Openstack cloud network setting during provisioning

https://bugzilla.redhat.com/show_bug.cgi?id=1138417

During openstack provisioning the cloud network needs to be
set only if there are more than 1 available cloud networks.
It looks like this method and `MiqProvisionWorkflow#validate_placement` are very close to copy 'n' paste.  I wonder if a small amount of refactoring in `MiqProvisionWorkflow#validate_placement` and this method could allow them to use the same base set of base validation cases.  And, only override the specific things that need to be overridden.

That might require some inordinate amount of restructuring to accommodate the successive `return nil if ...` statements.