Add support for corporate deployments where the root user installed an old version of Dask, with an associated /etc/dask/dask.yaml, e.g.

distributed:
   p2p:
     disk: false

while the local user deployed a newer version of dask in a local environment with its own config, e.g. ~/.config/dask/dask.yaml

distributed:
   p2p:
     storage:
        disk: false

In this use case, suppress the warning that the legacy config contains a deprecated key, as it's been explicitly overwritten by the local config.

In case of deleted keys: keep raising ValueError when the user sets a deprecated key (with no valid replacement) with dask.config.set, but just emit a warning if the obsolete key is in a config file. Also offer the user to override the config file, as they may be unable to directly modify the offending file due to permissions, as explained above.

(At the moment of writing there aren't any deleted keys in dask or distributed; not sure about third-party packages that piggy-back on the same config system).