Port 5000 for image-registry is not firewall/proxy friendly

[Nobuto Murata]

At this moment, kubernetes-master charm uses image-registry.canonical.com:5000/cdk as "image-registry" by default:
https://jaas.ai/u/containers/kubernetes-master/700#charm-config-image-registry

The port 5000 is not firewall/proxy friendly since the common rule for egress is usually port 80 for HTTP and port 443 for HTTPS only, then outgoing 5000 tends to be blocked in enterprise environments.

Now that image-registry.canonical.com listens on 443 too, it might be a good idea to migrate the port in the charm config as well. Here is the Canonical internal RT which added port 443:
https://portal.admin.canonical.com/C119961/

The migration strategy would need a careful review as some might have explicit whitelist of the pairs of destination host and port, so
1. call out the change in the release/upgrade notes clearly, or
2. some sort of mechanisms not to touch existing deployment and apply the new value to fresh deployments only
would be nice to have.

[George Kraft (cynerva)]

Added calico and canal since they also have charm configs that reference image-registry.canonical.com:5000.

[Nobuto Murata (nobuto)]

https://github.com/charmed-kubernetes/charm-kubernetes-master/pull/50
https://github.com/charmed-kubernetes/layer-calico/pull/45
https://github.com/charmed-kubernetes/layer-canal/pull/42