Add support for OVS l2 agent in XS/XCP domU.

Add support for OVS l2 agent in XS/XCP domU.

 * Config doc: http://wiki.openstack.org/QuantumDevstackOvsXcp

 * The Open vSwitch agent needs to be deployed on domU but
   interact with a dom0 bridge.
 * Add a root wrapper and associated XenAPI plugin to allow the
   agent to execute networking commands against dom0 from domU.
 * Update ovs_lib mac address discovery to use ip_lib to
   ensure that discovery works even for bridges not local to
   the agent (i.e. dom0 bridges).  A bridge configured with
   a dom0 root wrapper will execute ip link on dom0.
 * Update ip_lib to use a root helper by default to ensure that
   the 'ip' command will execute on dom0.
 * Remove obselete rpm spec and installer for dom0 agent.
 * Credit where credit is due - the XenAPI plugin and its
   packaging were largely copied from nova.
 * Supports blueprint xenapi-ovs

Change-Id: I7795446ee1267712c896f5cb3401f84fb1763ce7