Add versioning to CephFSVolumeClient interface
As we add features and change things in CephFSVolumeClient, we will need to add a mechanism for users of the module to declare what version they expect to see, and query whether this module can support that.
I don't think we necessarily need everything to be exhaustively backwards compatible, the key thing is that it should raise an error when an incompatible version is in play, rather than having some unexpected behaviour.
There are three entities with potentially three different versions: the Ceph cluster, the CephFSVolumeClient module, and the user (i.e. Manila). What I'm talking about versioning in this ticket is the CephFSVolumeClient<->Manila part.