object-ify flavors manager side of the RPC

This change completes the conversion of flavors to use objects instead
of direct database access. The rpcapi sends objects for the new version
and primitivized data is sent for older versions.

Related to blueprint kilo-objects

Change-Id: Ib876f276cff99a21c5bca1c9f062a6c221a783e8
Merge "object-ify flavors manager side of the RPC"