Return IPV4 openstack VM addresses

OVH have recently started providing IPV6 addresses along with their IPV4 addresses for new VMs. ref: http://travaux.ovh.net/?do=details&id=23185

Unfortunately, our logic for determining the public IP of a new VM was too simplistic, and so it broke when this change was rolled out.

This PR updates openstack_utils.get_server_public_address() and related tests to:

• Adds an optional ip_version parameter, default 4.
• Returns the first address found which matches the requested ip_version.

Reviewer

• @bdero