Drop RpcProxy usage from l2population code

This patch removes usage of the RpcProxy compatibility class from the
l2population code.  The equivalent oslo.messaging APIs are now used
instead.

Part of blueprint drop-rpc-compat.

Change-Id: Ie6d45f1871c2dba603bc17851b753bb960a9ca2b
Merge "Drop RpcProxy usage from l2population code"