Drop RpcProxy usage from LBaaS code

Update the LBaaS code to stop using the RpcProxy compatibility class.
The equivalent direct usage of oslo.messaging APIs are now used
instead.

Part of blueprint drop-rpc-compat.

Change-Id: I381394507e4f2daf6d774f70087fef8833c9bab5
Merge "Drop RpcProxy usage from LBaaS code"