Fix Metering doesn't respect the l3 agent binding

This patch fix the issue by changing the call to
find the plugin which handles the l3 which is now
the l3_router service plugin instead of the old mixin.

Also change the unit tests to use the l3 service plugin
instead of the l3 mixin and refactor the rpc callbacks
part.

Co-Authored-By: Ala Rezmerita <ala.rezmerita@cloudwatt.com>
Closes-bug: #1257354
Change-Id: Ide26f825005fa63cd3fcc75fa91fffb947e0be7a
Merge "Fix Metering doesn't respect the l3 agent binding"