Configure local registries to proxy docker hub

Added the REGISTRY_PROXY_REMOTEURL parameter to the registry
creation script, so that the registry will mirror images from
Docker Hub (while still being able to host custom-built images).
Updated the Multinode guide to reflect this.

Change-Id: Ib789241fd2947b06fb0dc5732a0bf28431e1403d
Closes-Bug: #1647436

doc/multinode.rst

@@ -35,6 +35,10 @@ Configure Docker on all nodes
 .. note:: As the subtitle for this section implies, these steps should be
           applied to all nodes, not just the deployment node.
 
+The ``start-registry`` script configures a docker registry that proxies Kolla
+images from Docker Hub, and can also be used with custom built images (see
+:doc:`image-building`).
+
 After starting the registry, it is necessary to instruct Docker that it will
 be communicating with an insecure registry. To enable insecure registry
 communication on CentOS, modify the ``/etc/sysconfig/docker`` file to contain

tools/start-registry

@@ -5,4 +5,5 @@ docker run -d \
     --restart=always \
     -p 5000:5000 \
     -v registry:/var/lib/registry \
+    -e REGISTRY_PROXY_REMOTEURL=https://registry-1.docker.io \
     registry:2