Motivation for this change

Flannel can now use the kubernetes api as storage backend, instead of etcd. This is useful especially when deploying flannel with kubernetes where you - otherwise - either need to allow multi-user access to a single etcd instance or setup a secondary etcd instance just to hold flannel config. When configuring flannel with kubernetes as storage backend, you can isolate the access to etcd by authorizing only kube-apiserver.

backward compatibility: The default behavior is unchanged. Etcd will be used as storage backend, unless the new option services.flannel.storageBackend is set to kubernetes.

furthermore: the flannel test case fails on master. etcd refuses to startup, because it requires ip-adresses configured as listening adresses in order to determine which interfaces to bind to. Fixed that as part of this PR.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.