Don't leave charm files sitting in the cache

[babbageclunk]

Description of change

When the charmrepo downloads charm archives it stores them in a cache directory. On the controller this is unnecessary, since the archive is loaded into the blobstore (which is available to all of the controllers in HA, rather than just the one that downloaded the charm initially). Additionally, if a controller had had a large number of models that had been migrated away or otherwise removed, the charms used by those models would still be taking up space on the filesystem.

Remove the charm from the cache once it's been stored.

A cleaner way to do this would be to have the cache dir be an attribute of the CharmStore. The store would get a Cleanup method that would be called when the archive is finished with and would delete the charm dir. Then the controller code wouldn't set a cache dir at all - it would just be a temp directory, and would be removed when the charm was finished with. I haven't done that yet because I want to talk to @rogpeppe about what would work for other places that use the charm store.

Also includes a driveby to rename service to application in the apiserver/application unit tests.

QA steps

• Bootstrap a controller and add a model
• Deploy ubuntu
• Check that the charm cache directory (/var/lib/juju/charmcache) is empty on the controller.

Bug reference

Part of the fix for https://bugs.launchpad.net/juju/+bug/1579976

[babbageclunk]

$$merge$$

[jujubot]

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

[jujubot]

Build failed: Tests failed
build url: http://juju-ci.vapour.ws:8080/job/github-merge-juju/10329

[jameinel]

$$merge$$

[jujubot]

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