Generic rpc callback mechanism which could be reused

Generic rpc callback mechanism which could be reused

This is a publisher/subscriber messaging mechanism optimized
for agent consumption and server production without the need
of creating new rpc messages when new resources are introduced.

Oslo versionedobjects are the perfect match to ensure
cross version compatibility even if the published/subscribed
resources format change over time.

This is still a basic stub allowing get_info of the resources,
and the next change will introduce the RPC methods to call
get_info: I0ac8a009e781b6edb283d8634b1a2f047db092dc

The plugin is returning stub objects to be consumed from the
agent to test the basic behaviour until we have DB.

TODO: Update documentation, according to code changes,
      enforce versioned objects only doing deserial/serialization.

Co-Authored-By: Miguel Angel Ajo <mangelajo@redhat.com>
Co-Authored-By: Eran Gampel <eran@gampel.net>
Change-Id: I524cf5a14e99dc6bee4d4261557d98c75efa0809