Let's reuse the container image prepare default command to generate the
container prepare items rather than hard coding it in the
standalone_parameters.yaml. This is closer to what we would expect a
user to do for the overcloud or is automatically done during the
undercloud installation.
Change-Id: Ibdc8e7a034ee5a07c75fc1423a2266bc93d3fcb0
0d6d70d18d