BZ #1129227 - Set l3 external_network_bridge to ''.

https://bugzilla.redhat.com/show_bug.cgi?id=1129227

This is needed to properly use provider networks in newer versions of neutron.
Merge pull request #348 from jguiditta/bz1129227-br-ex

BZ #1129227 - Set l3 external_network_bridge to ''.