Drop RpcProxy usage from VPNaaS code

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

Part of blueprint drop-rpc-compat.

Change-Id: I4ff0bfe0b5e909bfe088f4059d85aa6366526dad
Merge "Drop RpcProxy usage from VPNaaS code"