Creates external bridge before starting ovs agent

The agent is currently involved in a race condition
where br-ex is created after it, but will still be
created before the agent searches for it in cases
where the intermediate resources are resolved quickly.

The agent will terminate immediately if the bridge
is not found.

Change-Id: Ie93289ce156f5c1d76bae7616b573107d16dd6aa
Fixes: bug #1190117