Use correct modify_append_tag key
Use correct modify_append_tag key

kolla_builder is setting modify_append_tag, not append_tag:
http://git.openstack.org/cgit/openstack/tripleo-common/tree/tripleo_common/image/kolla_builder.py#n127

Change-Id: If19e2f3f723e437b2213cacc2f4dfdbe2e0ffce0
Blueprint: container-prepare-workflow

openstack/tripleo-common,openstack/tripleo-common