Add configurable use of veths with Quantum+OVS.

 * This patch adds the ability to configure use of veths with
   OVS via the Q_OVS_USE_VETH variable.  The use of veths with OVS
   is required to support namespaces on RHEL.
 * Supports bug 1171727

Change-Id: I5f557d659684ead99a3e5e2b4df787699d9d3f05
Merge "Add configurable use of veths with Quantum+OVS."