qa_crowbarsetup: Fix magnum service image name
qa_crowbarsetup: Fix magnum service image name

Commit 7835272aa7 added the ability to use locally cached images but
changed the magnum-service-image name wrongly. Fixed with this commit.

aspiers/automation,gosipyan/automation,aspiers/automation,SUSE-Cloud/automation,aspiers/automation,SUSE-Cloud/automation,SUSE-Cloud/automation,SUSE-Cloud/automation,gosipyan/automation,aspiers/automation,gosipyan/automation,gosipyan/automation