Before this change, if you ended up writing a middle layer whose parent is not in the database, the manifest would fail to rewrite. We now just lookup the parent image in the manifest given to us, ignoring whether it is in the database or not (as it doesn't actually matter if not present; it'll be created if necessary).
Also adds some tests to registry tests for V1 stuff.
Note: All *registry* tests currently pass, but as verbs are not yet converted, the verb tests in registry_tests.py currently fail.
