Add Krew command for recovering the cephcluster CR after accidental deletion

[travisn]

As seen in the Rook disaster recovery guide, restoring the cephcluster CR after deletion is a very tedious and accident-prone process. Similar to the command just added for mons restore-quorum in #61, we could add a command such as:

kubectl rook-ceph undelete cephcluster [name]

Where the name could be rook-ceph by default, which is also the default for the namespace, and is the default in our example deployments.

We could also have undelete commands for other CRs, which is why we might consider undelete as a top level command.