Add limited support for Quantum+OVS on XS/XCP.

 * Add priliminary support for running the OVS L2 and DHCP agents
   in domU:
   * Configure Nova to use the correct vif driver and integration
     bridge.
   * Configure the ovs agent to target the dom0 integration bridge.
   * Install a xapi plugin supporting dom0 execution of ovs agent
     commands.
 * Config doc: http://wiki.openstack.org/QuantumDevstackOvsXcp
 * Supports blueprint xenapi-ovs

Change-Id: If5ab07daab1dc3918004eb4bfb6fed6cab0a71fd
Merge "Add limited support for Quantum+OVS on XS/XCP."