Scope secgroup rpc api using a messaging namespace

Scope secgroup rpc api using a messaging namespace

This patch scopes the agent to plugin security group rpc interface
using a messaging namespace.  Right now some plugins expose several
interfaces via the default namespace.  This effectively means they are
a single API and should be managed with a single version stream.  It's
much more managable to just treat these as separate interfaces and
this change makes that explicit and functionally true.  Now when a
method is invoked, the only classes considered for handling that
request will be ones marked with the right namespace.

Part of blueprint rpc-docs-and-namespaces.

Change-Id: Iaee934646c9da7d32968406a583a5718fffc893b