Bring up interface when enslaving to a bridge

Right now, if the interface to be used for a nova-network bridge is
not already up, nova-network will enslave it and leave it down,
which is not very helpful. Since an interface for this purpose is
likely to be left unconfigured by the base OS, expecting it to be
up is not ideal.

Change-Id: I14564c0bcb55b9dd427ccaa8fb217e38121d4b71
Closes-bug: 1228183
Merge "Bring up interface when enslaving to a bridge"