Adds support for L3 routing/NAT as a service plugin

- Adds L3 routing/NAT service plugin
- Removes L3 routing/NAT from ML2 plugin
- Moves "router:external" attribute to new extension "External-net"
- Introduces separate RPC topic for L3 callbacks from L3 agent

Implements: blueprint quantum-l3-routing-plugin

Change-Id: Id9af10c2910f9a1730b163203a68d101ffc3b282