The docker registry v1 and v2 versions have completely different
authentication methods that need handling. The v2 OAuth scheme
requires modifying request headers and re-trying requests after
getting an auth token. Introduce a pluggable framework for auth
can be hooked into the _get_url() method to deal with the request
and response objects, as well as errors.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>