Implementation of 2nd phase of provider extension for openswitch

Implementation of 2nd phase of provider extension for openswitch

Enhances openvswitch plugin to support flat networks and VLANs on
multiple physical networks via the provider extension. Implements
blueprint provider-networks.

See http://wiki.openstack.org/ConfigureOpenvswitch for
configuration and usage details.

A devstack patch to support the updated openvswitch configuration
variables is at https://review.openstack.org/#/c/11418/.

Change-Id: Ic86b6f3b2e354c7d60bc2c330b334c23d349bc29