Embrane Plugin Support

Implements blueprint embrane-plugin-support

This commit implements Embrane's Neutron plugin installation support
in Devstack.
This is an extension of the openvswitch installation module, which is
used by the main plugin, and enables configuration by localrc

Change-Id: Ia4824f8d2300bcdce170d226145bbce6088f1557
Merge "Embrane Plugin Support"