Description of problem:  Before updating the overcloud we need to update all the image in the local registry in the undercloud.

The command used in OSP14 for doing so is:

  openstack overcloud external-update run --tags container_image_prepare

That command doesn't work anymore in osp15.  First issue is that the external tasks are just ignored, second issue is that the containers are downloaded inside the mistral-executor container and not on the undercloud.

This result in a failure during overcloud update where we can't find the new images in the undercloud registry.

Version-Release number of selected component (if applicable): Beta-1.0/trunk to passed_phase2 

The workaround is to use another command directly from the undercloud:

sudo openstack tripleo container image prepare \
    -e ~/containers-prepare-parameter.yaml \
    --output-env-file containers-prepare-parameter.yaml

But we'd rather have the same interface than in osp14.