Port all rpcapi modules to oslo.messaging interface

Add a temporary nova.rpcclient.RPCClient helper class which translates
oslo.messaging.rpc.RPCClient compatible calls into calls on a RpcProxy
object.

Use this new class to port all of the rpcapi modules over to the new
RPCClient so that the final port of Nova over to oslo.messaging will be
smaller and easier to review.

This patch contains no functional changes at all, except that all client
side RPCs go through this temporary helper class.

blueprint: oslo-messaging
Change-Id: Iee86c36bcc474a604993618b8a2255af8c3d2f48
Merge "Port all rpcapi modules to oslo.messaging interface"