Allow push/save without image reference

If the user uses the image ID when saving to either docker-archive
or oci-archive, then do not save a reference in the manifest/index.json.
If the user chooses to push without an image reference, i.e <transport>:<path>
it should be valid and succeed.

Signed-off-by: umohnani8 <umohnani@redhat.com>

Closes: #782
Approved by: rhatdan
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