Ensures that the Linux Bridge Plugin runs with L3 agent

Ensures that the Linux Bridge Plugin runs with L3 agent

Fixes bug 1048968

1. The external interface for the linux bridge will be created at run time.
The configuration parameter external_network_bridge should be set as empty.
That is, "external_network_bridge = ". The link between the network and
the physical interface should be done via the provider network parameters.
2. The plug operation for the linux bridge replaces the interface name
prefix with tap. We need to ensure that the prefix length is 3 characters.

Change-Id: Ib8e26bc1f603a1aff9560b4a40110a6824db5921