quay-pruner/build/prune-quay.py: handle new "short tags"

quay-pruner/build/prune-quay.py: handle new "short tags"

There are two new tags proposed for each image: "branchname" and "full
sha1".  Branchname is currently being created.  When pruning image tags,
we need to prune the associated short tags as well.

This also fixes a current bug where, because branchname tags are being
created, the current unaware prune script deletes them because it
treats "can't parse tagname" as equivalent to "can't find in shaman".

Signed-off-by: Dan Mick <dan.mick@redhat.com>