Scalable backup service - Liberty compatibility

Scalable backup service - Liberty compatibility

To support rolling upgrades we need to make sure that Mitaka's services
are running fine with Liberty's. It gets complicated with backups as
we've strongly reworked them. Main difference is that Mitaka c-bak can
handle backup/restore of any volume and Liberty was restricted to
operate only on volumes placed on the same node.

Now when running in version heterogeneous environment we need to use old
way of backup jobs scheduling and switch to new one (round robin) only
when everything is running Mitaka.

This commit implements that by adding a dummy backup RPC API version
(1.3) that marks the beginning of scalable backups era. Jobs are
scheduled the new way only if every c-bak reports that (or higher)
version.

There are also small changes to volume.rpcapi - to fail fast if some
c-vol services aren't supporting new calls required by scalable backups
feature. This allows us to error out backups with proper message when
upgrade was done in an improper way (in Mitaka we require c-vols to be
upgraded before c-baks).

This commit also includes small changes to CinderObjectSerializer to
block tries to "forwardport" an object when sending it over RPC. If a
service receives an older object it should handle it explicitly.

Related-Blueprint: scalable-backup-service
Co-Authored-By: Michal Dulko <michal.dulko@intel.com>
Change-Id: I45324336ba00726d53cfa012e8bd498868919a8c