Skip FWaaS config mismatch check if RPC method is unsupported

In this commit FWaaS config check skip is skipped
if neutron server does not support this API.

Commit d6f014d introduced FWaaS config mismatch check between
server and agent. It added a new RPC method get_service_plugin_list
and bumped l3-agent RPC version to 1.3, but this version RPC is
only supported by L3 router service plugin and
it breaks existing plugins using L3 router mixin.

Bumping l3-agent RPC version requires detailed investigation on all
affected plugins and it can be done by plugin maintainer later.

Change-Id: I388a24b0c6a507203674ef108bb14cea0534f98c
Closes-Bug: #1353309
Merge "Skip FWaaS config mismatch check if RPC method is unsupported"