Allow removal of store charms

[babbageclunk]

Description of change

Charm-store charms should be removed when they're no longer needed in
the same way as local charms. Otherwise the associated storage records in
the blobstore never get removed, which means that the DB will contain every
charm ever deployed on the controller, regardless of whether it's currently used.

It's not clear why this was prevented - the comments don't really
explain. Experimentation shows that we can reinstate removed charms if
they're subsequently deployed again. This seems like a better trade-off

• they're cached locally if there's anything using them (so adding units
  doesn't require getting the charm again) but they're not kept around
  forever, taking up space that isn't ever returned. (See the bug comments
  for more discussion.)

QA steps

• Bootstrap a controller with two models.
• Deploy a charm from the store (eg ubuntu) in both models.
• Remove one model.
• Check that you can still add units to the application using the charm in the other model.
• In the mongo DB you can see that there's only one record for the charm in the managedStoredResources collection: db.managedStoredResources.find({"path": {"$regex": "charm"}})

Bug reference

Part of fixing https://bugs.launchpad.net/juju/+bug/1579976

[babbageclunk]

!!chittychitty!!

[howbazaar]

Have you tried redeploying the same charm into the same model once it has been removed? Because that is what the comment seems to indicate.

[babbageclunk]

Oh, yes - I've tried that too.

[babbageclunk]

$$merge$$

[jujubot]

Status: merge request accepted. Url: http://juju-ci.vapour.ws:8080/job/github-merge-juju