Added RpcProxy class

This class is intended for easy and iterative migration from
oslo-incubator RPC implementation to oslo.messaging. It will adapt
oslo.messaging API to old oslo-rpc interfaces.

Migrated all affected classes to the new RpcProxy class.

blueprint oslo-messaging

Change-Id: I1563c13b470b61eebc982ee81efee47732943d48
Merge "Added RpcProxy class"