To accomodate running in a production-simulation environment,
make it safe to run this role on a host before docker is installed.
This also adds support for the new dual-registry configuration
that run-buildset-registry uses.
This removes the region-local proxy from the registry-mirrors
configuration. Because the buildset registry acts as a pull-through
proxy, the region-local proxy won't be used even if we did include it.
Instead, we should update the run-buildset-registry role to proxy
to the region-local proxy if present.
Change-Id: I21011a3708f17ee61afd0034d90d75e8dc885575
42df455705