Use `versionutils.is_compatible` for Nova Objects

Oslo's check is designed to support version checking for a number of different
existing use-cases, including this one.

Related-Oslo-Commit: d7d74a752e34d5249fa47c8e8252da491d46012a
Change-Id: I8e03c7781330b02deb7af815d68538b5c0245c4d
Merge "Use `versionutils.is_compatible` for Nova Objects"