qa_openstack: allow to override $mirror
qa_openstack: allow to override $mirror

in order to run this in other places

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