Revert changes removing OVSBridge return

This reverts changes from this review [1] which incorrectly removed
the return of the OVSBridge object when calling BaseOVS.add_bridge.
This broke all functional tests relying on this return.

[1] https://review.openstack.org/#/c/71142/

Related-Bug: #1301363

Change-Id: If32e155cc2bd368b2e842f14b1d1da0827ccfeff
Merge "Revert changes removing OVSBridge return"