Enable running the registry in a mode where authentication is required
for pulling images. This could be useful in an environment where even
an intermediate or buildset registry should require authentication to
pull images. Or it could make this more useful as a general registry
(that's not a priority use case for this project, but this doesn't add
much complexity).
If a "read" level user is specified, then we assume that anonymous
read access should not be allowed.
Change-Id: I1455a1031590ff0206a4b6da0d8c08093cf0e3cd
b635c65cf3