Don't use docker override in scenario012 standalone ironic
Don't use docker override in scenario012 standalone ironic

It's set in job definition:
https://github.com/openstack/tripleo-ci/blob/master/zuul.d/standalone-jobs.yaml#L585

Change-Id: Ia5285e530417c50bded380593877b5614c5784ef

openstack/tripleo-heat-templates,openstack/tripleo-heat-templates