Currently when pulling/pushing images we default to the format
of the source image. When Building images we default to OCI.
Customers have asked us to allow them to force a specific image
type when pushing to a registry.
We already have a flag to building images.
This PR adds image_default_format to define the format to be used
by all tools create/building/pulling and pusing images.
Signed-off-by: Daniel J Walsh <dwalsh@redhat.com>