Replace deprecated params in prepare
Replace deprecated params in prepare

Update prepare should only run the stack update
and Download the playbooks required to run the
update. The config_download arg was deprecated
by:
https://review.opendev.org/c/openstack/python-tripleoclient/+/803610

As such, we need to use stack_only = True to
ensure the update prepare is only running the
stack update, and not the entire deployment.

Change-Id: If7e8e25aba35ec8a00b2190abee2c6a8784b291e

openstack/python-tripleoclient