These tests have been stable for a few months now. Reduce to a weekly
cadence.

Release note: None

Long-lived disk snapshots can drastically reduce testing time for scale
tests. Tests, whether run by hand or through CI, need only run the
long running fixture generating code (importing some dataset, generating
it organically through workload, etc.) once snapshot fingerprints are
changed, fingerprints that incorporate the major crdb version that
generated them.

Here's an example run that freshly generates disk snapshots:

    === RUN   admission-control/index-backfill
    03:57:19 admission_control_index_backfill.go:53: no existing snapshots found for admission-control/index-backfill (ac-index-backfill), doing pre-work
    03:57:54 roachprod.go:1626: created volume snapshot ac-index-backfill-0001-vunknown-1-n2-standard-8 (id=6426236595187320652) for volume irfansharif-snapshot-0001-1 on irfansharif-snapshot-0001-1/n1
    03:57:55 admission_control_index_backfill.go:61: using 1 newly created snapshot(s) with prefix "ac-index-backfill"
    03:58:02 roachprod.go:1716: detached and deleted volume irfansharif-snapshot-0001-1 from irfansharif-snapshot-0001
    03:58:28 roachprod.go:1764: created volume irfansharif-snapshot-0001-1
    03:58:33 roachprod.go:1770: attached volume irfansharif-snapshot-0001-1 to irfansharif-snapshot-0001
    03:58:36 roachprod.go:1783: mounted irfansharif-snapshot-0001-1 to irfansharif-snapshot-0001
    --- PASS: admission-control/index-backfill (79.14s)

Here's a subsequent run that makes use of the aforementioned disk
snapshot:

    === RUN   admission-control/index-backfill
    04:00:40 admission_control_index_backfill.go:63: using 1 pre-existing snapshot(s) with prefix "ac-index-backfill"
    04:00:47 roachprod.go:1716: detached and deleted volume irfansharif-snapshot-0001-1 from irfansharif-snapshot-0001
    04:01:14 roachprod.go:1763: created volume irfansharif-snapshot-0001-1
    04:01:19 roachprod.go:1769: attached volume irfansharif-snapshot-0001-1 to irfansharif-snapshot-0001
    04:01:22 roachprod.go:1782: mounted irfansharif-snapshot-0001-1 to irfansharif-snapshot-0001
    --- PASS: admission-control/index-backfill (43.47s)

We add the following APIs to the roachtest.Cluster interface, for tests
to interact with disk snapshots. admission-control/index-backfill is a
placeholder test making use of these APIs.

  type Cluster interface {
      // ...

      // CreateSnapshot creates volume snapshots of the cluster using
      // the given prefix. These snapshots can later be retrieved,
      // deleted or applied to already instantiated clusters.
      CreateSnapshot(ctx context.Context, snapshotPrefix string) error

      // ListSnapshots lists the individual volume snapshots that
      // satisfy the search criteria.
      ListSnapshots(
        ctx context.Context, vslo vm.VolumeSnapshotListOpts,
      ) ([]vm.VolumeSnapshot, error)

      // DeleteSnapshots permanently deletes the given snapshots.
      DeleteSnapshots(
        ctx context.Context, snapshots ...vm.VolumeSnapshot,
      ) error

      // ApplySnapshots applies the given volume snapshots to the
      // underlying cluster. This is a destructive operation as far as
      // existing state is concerned - all already-attached volumes are
      // detached and deleted to make room for new snapshot-derived
      // volumes. The new volumes are created using the same specs
      // (size, disk type, etc.) as the original cluster.
      ApplySnapshots(
        ctx context.Context, snapshots []vm.VolumeSnapshot,
      ) error
  }

This in turn is powered by the following additions to the vm.Provider
interface, implemented by each cloud provider.

  type Provider interface {
      // ...

      // CreateVolume creates a new volume using the given options.
      CreateVolume(l *logger.Logger, vco VolumeCreateOpts) (Volume, error)

      // ListVolumes lists all volumes already attached to the given VM.
      ListVolumes(l *logger.Logger, vm *VM) ([]Volume, error)

      // DeleteVolume detaches and deletes the given volume from the
      // given VM.
      DeleteVolume(l *logger.Logger, volume Volume, vm *VM) error

      // AttachVolume attaches the given volume to the given VM.
      AttachVolume(l *logger.Logger, volume Volume, vm *VM) (string, error)

      // CreateVolumeSnapshot creates a snapshot of the given volume,
      // using the given options.
      CreateVolumeSnapshot(
        l *logger.Logger, volume Volume, vsco VolumeSnapshotCreateOpts,
      ) (VolumeSnapshot, error)

      // ListVolumeSnapshots lists the individual volume snapshots that
      // satisfy the search criteria.
      ListVolumeSnapshots(
        l *logger.Logger, vslo VolumeSnapshotListOpts,
      ) ([]VolumeSnapshot, error)

      // DeleteVolumeSnapshot permanently deletes the given snapshot.
      DeleteVolumeSnapshot(l *logger.Logger, snapshot VolumeSnapshot) error
  }

Since these snapshots necessarily outlive the tests, and we don't want
them dangling perpetually, we introduce a prune-dangling roachtest that
acts as a poor man's cron job, sifting through expired snapshots
(>30days) and deleting them. For GCE at least it's not obvious to me how
to create these snapshots in cloud buckets with a TTL built in, hence
this hack. It looks like this (with change to the TTL):

    === RUN   prune-dangling
    06:22:48 prune_dangling_snapshots_and_disks.go:54: pruned old snapshot ac-index-backfill-0001-vunknown-1-n2-standard-8 (id=7962137245497025996)
    06:22:48 test_runner.go:1023: tearing down after success; see teardown.log
    --- PASS: prune-dangling (8.59s)

Subsequent commits will:
- [ ] Fill out admission-control/index-backfill, a non-trivial use of
      disk snapshots. It will cut down the test time from >4hrs to <25m.
- [ ] Expose top-level commands in roachprod to manipulate these
      snapshots.

Release note: None