Drop RpcProxy usage from cisco.cfg_agent

Drop usage of the RpcProxy compatibility class from cisco.cfg_agent.
The equivalent direct usage of oslo.messaging APIs is now used
instead.

Part of blueprint drop-rpc-compat.

Change-Id: I3ec71b3689adf7e96cdc2fdbf501c5cc37a9e0e4
Merge "Drop RpcProxy usage from cisco.cfg_agent"