DeferredBridge to allow add_tunnel_port passthru

The DeferredBridge should allow add_tunnel_port
method as passthrough in addition to add_port
and delete_port. L2Pop uses add_tunnel_port to
dynamically establish tunnel endpoints on cloud
nodes.

Closes-Bug: #1353885

Change-Id: I2e6d82ee79814e500604f4951e7d89eab662047a