Add ML2 plugin support

This patch aims to support:
- ml2 configuration for Open-vSwitch & Linux Bridge plugins
- mechanism drivers: flat, gre, vlan vxlan
- l2 population support for OVS & linux bridge plugins

implement blueprint ml2-support
Cherry-picked from https://review.openstack.org/#/c/48289/

Change-Id: I9a4224f899e95411c509f583bf04dfc4dd0de25b
Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>