Change charms to use juju-http{s}-proxy model settings

[Cynerva]

Fixes https://bugs.launchpad.net/layer-docker-registry/+bug/1832336

Built on the work of @VariableDeclared from #138 (thanks!).

I tested this by building the docker-registry charm with these changes. I deployed it and confirmed the following:

1. When no charm or model proxy configs are set, the service file renders with no proxy.
2. When charm proxy configs are set, they are included in the service file.
3. When model proxy configs are set, they are included in the service file.
4. When both charm and model proxy configs are set, the charm configs are used.
5. juju-no-proxy CIDRs are expanded correctly before being rendered in the service file

[tvansteenburgh]

Thanks for the thorough description of the testing you did.