Adding feature of setting bridge-id on external bridge

Because by default external bridge comes without any OVS metadata,
neutron-servers mark all external ports, plugged into this bridge as
DOWN, but network connections works properly. This patch fixes this
issue.FIXME: neutron-server daemon restart is needed on controller
nodes to apply this configuration.
Merge pull request #15 from abubyr/stable/havana

Adding feature of setting bridge-id on external bridge