Verify network interfaces are in requested order

There is no Tempest test verifying that the order of multiple networks passed
to an instance during its creation is preserved within the instance
(https://review.openstack.org/#/c/85711/). This change introduces a new test
in test_create_server to verify this feature.

Change-Id: I3622b9e8dfbbf73eb9a0a69d2094b75e4934fe17
Merge "Verify network interfaces are in requested order"