Add L3 service in service_plugins param

This service plugin implements the L3 routing functionality (resources
router and floatingip) that in earlier releases before Havana was provided by
core plugins (openvswitch, linuxbridge, ... etc).

Core plugins can now choose not to implement L3 routing functionality
and instead delegate that to the L3 routing service plugin.

This patch aims to add the new service in the param documentation.

Change-Id: I03fe52706f2857d42baf01db8362804c597f5155
Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>
Merge "Add L3 service in service_plugins param"