Stop using a hardcoded cirros image name
Stop using a hardcoded cirros image name

In the tests we have the cirros image name hard-coded.
This requires us to update it every time the test image is updated.
Instead, now we list the images and take the first one (which is safe enough for
the circumstances).
Anyway, if a specific image is required, it may still be requested in the
'vm_image' argument.

Change-Id: I0aae2a01ca5fd67bd828bb8daf6df466c60acff2

openstack/dragonflow,openstack/dragonflow,openstack/dragonflow