Implement MidoNet Neutron plugin for Havana

Implement L2, L3, security groups, metadata server support for
MidoNet Neutron plugin for Havana.

blueprint https://blueprints.launchpad.net/neutron/+spec/midonet-plugin-havana

Change-Id: I0dd1a2ca17d760443c4c7a464a66b6d0a2cf194a