MD5 image checksums have long been supersceeded by the use of a
``os_hash_algo`` and ``os_hash_value`` field as part of the
properties of an image.
In the process of doing this, we determined that checksum via
URL usage was non-trivial and determined that an appropriate
path was to allow the checksum type to be determined as needed.
Change-Id: I26ba8f8c37d663096f558e83028ff463d31bd4e6
32df26a22a