Add OS::Neutron::RouterL3Agent for router.py

This resource supports part of "agent management and scheduler
extensions" in Neutron. It associates router with l3-agent.

see related-bug: #1245366

Closes-bug: #1259845
Change-Id: I781645e62beb4fe60e064d61ad298834e551142d
Merge "Add OS::Neutron::RouterL3Agent for router.py"