When a vif will be plugged into an OVS bridge, ensure that the bridge
exists during the plugging operation. In most cases, the OVS bridge will
already exist at the time of port creation, so this will be a no-op.
However, the OVS agent implementation for vlan-aware VMs will require
the os-vif plugin to create the OVS bridge if it doesn't already exist,
because vifs that correspond to trunk ports won't be directly plugged
into br-int.
Change-Id: I3cb61a5e842177a26f38c85d7782ecced4ac07da
3d62d8e23c