juju upgrade-charm does not have a syntax for moving back to a charmstore resource

[Richard Harding]

If I have a charm that I deploy, and update the resource with a local copy to test something, there's no method to get back to tracking the upstream charmstore resource revision chain. juju upgrade-charm only accepts a local file path as a resource identifier.

We should add some syntax to be able to point the --resource flag back at a remote URL so that the deployed application can be updated to move back to the store based resource chain.