This registry can be accessed through proxies that run on each node
listening on port 5000. We send the proxy images to the nodes directly
to avoid requests that hit the network during cluster launch. For now,
we continue to pull the registry itself over the network, especially
given its large size (we should be able to dramatically shrink the
image). On GCE we create a PD and use that for storage, otherwise we
use an emptyDir. The registry is not enabled outside of GCE. All
communication is currently plain HTTP. In order to use SSL, we will
need to be able to request a certificate/key from the apiserver signed
by the apiserver's CA cert.
