When enabled provider networks; the neutron-openvswith-agent-xenapi
should use proper bridge_mappings which should have dom0's bridges
where the VMs' vifs actually connected to for the interfaces defined
in the configure option of *neutron_external_interface*.

Depends-On: I9a6bebe19ed488bb2173d5dc2daa14e236411243
Change-Id: I44f59c69d25b8400e1b936fcdf8f21b5c4168f1f