Allow to parameterize image name in tests
Allow to parameterize image name in tests

Add the ability to pass an image name using an env
var, to provide more flexibility on image testing.

Change-Id: Iddb2ab3efb5746a070e147ccc2180915c788c5c2

openstack/magnum,jay-lau/magnum,ArchiFleKs/magnum,openstack/magnum,ArchiFleKs/magnum