VMware: Network fallback in case specified one not found

In the case that opqaue network does not match the bridge ID then
we will fall back on a predefined network (configurable by the admin
via 'integration_bridge' in the 'vmware' section).

Fixes bug 1225002

Change-Id: Icfe2cb40feada33e5d86e669e1c88f2ce0dd1c5d