Extract the non-RHUI specific package set and image configuration into a
common definitions, which will be used by both image types.
Redefine the package sets and default image configuration used by both
image types to inherit from a common definition.
Regenerate image manifests for RHEL-8 / c8s `vhd` and `azure-rhui`
images.
There is no change in the resulting manifest for the `azure-rhui` image
type. However there are substantial changes to the `vhd` image
definition, which is now almost identical to the `azure-rhui` image
type, to provide consistent experience regardless if using RHUI or not.
The default partition table used by the `vhd` image type has been kept
as it was before, since there is yet no consensus on what size to
standardize for both image types.
