Fix fallback when Quantum doesn't provide a 'vif_type'

In

  commit 4babf7d7e12f35c137148727842f7af28312e858
  Author: Daniel P. Berrange <berrange@redhat.com>
  Date:   Fri Jan 4 12:22:20 2013 +0000

    Make Quantum plugin fill in the 'bridge' name

We changed libvirt VIF driver to pull 'bridge' out of the
VIF model. This only worked for new Quantum drivers that
provide the 'vif_type' portbinding metadata though. This
patch effectively reverts part of that patch, so we have
a combination of the new & old method for determining
bridge names. The libvirt VIF driver will look at the
network['bridge'] field first, and if missing will fallback
to its previous logic which hardcoded bridge names.

Bug: 1100199
Change-Id: I5fd66b0303159c42bd227e97ddab381b2b67670c
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Merge "Fix fallback when Quantum doesn't provide a 'vif_type'"