Revert "remove external_network_bridge option"

This reverts commit faaf96bfb15c5f4c45a72b149dc6fe1e1f907a71.

Ironic jobs were still using this option, it needs to be
switched to an alternative first.

Change-Id: I1683d7cfa81f5fe2497cc7045e87f8b20fed4968
2 files changed