CONTAINER_IMAGE_PREPARE_PARAM needs to be patched downstream for
different build pipelines, but this patching is hard to maintain.
This change breaks out this information into the file
container_image_prepare_defaults.yaml. Downstream (or stable branches)
can then set their image defaults by replacing all of
container_image_prepare_defaults.yaml in packaging.
Storing the unparsed yaml in CONTAINER_IMAGE_PREPARE_PARAM_STR also
gives the potential for the "openstack tripleo container image prepare
default" command to write out a file which preserves the comments,
which will help deployers understand what each value means.
Change-Id: I94d6d0e3f596a95374075f45a710869818c7083e
90a1891e7a