This commit adds an option for controlling request of compaction to
etcd3 from apiserver. There is a situation that apiserver cannot fully
own its etcd cluster (e.g. sharing it with canal). In such a case,
apiserver should have limited access in terms of etcd's auth
functionality so it don't have a priviledge to issue compaction
requests. It means that the compaction requests should be issued by
other component and apiserver's compaction requests are needless.
For such use cases, this commit adds a new flag
`storagebackend.Config.CompactionInterval`. If the flag is non 0,
apiserver issues the compaction requests like current behaviour (the
default is 5 minutes). If it is 0, apiserver doesn't issue the
requests. It can be configured with a newly added option of apiserver
`--etcd-compaction-interval`.
Kubernetes-commit: 87d4d3e92be6b93517f189082b0451cee6957ee5
a7bf68f0d6