qa_openstack: allow to use repo mirror
qa_openstack: allow to use repo mirror

by setting repomirror=http://ownserver/path

also reduces duplication of constant URL string

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