Don't schedule placeholder resource cleanup

[mjs]

Placeholder resources aren't stored in the blobstore and therefore don't need cleaning up. The fix has been made in two ways:

1. Cleanups are no longer scheduled for placeholder resources.
2. The resource cleanup code now ignores cleanups without a storage path set. This is a useful backstop but also fixes stray cleanups in existing deployments.

Fixes https://bugs.launchpad.net/juju/+bug/1649179

There was no testing at all for resource cleanup so this has also been rectified.

QA

Deployed the etcd charm as this uses a placeholder resource, and then removed it. Previously this would result in errors in the logs as the cleaner worker attempted to remove a resource which wasn't stored.

[anastasiamac]

This reads funny, but maybe only to me...
Would it not be simpler to start with:
...
if err != nil {
if errors.isNotFound(err) {
return false
}
r.Close()
return true
}
c.Fatalf(...)
return false
}
...

[mjs]

Fair enough, I'll rearrange.

[mjs]

$$merge$$

[jujubot]

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