Fix instance creation when a network with no subnet is used

It is possible with OpenStack Networking to create a network with no
subnet attached to it.
When creating an instance associated to such a network, the engine would
raise a 'list index out of range' exception because the instance has no
IP address on the network.
The issue would also happen with a loadbalancer referencing the same
type of instance.

Change-Id: I6122efb90877d8020added44aee8802eb8a80297
Fixes: bug #1187023
Merge "Fix instance creation when a network with no subnet is used"