Update etag of parent dir when adding/removing ext storage mount points

See https://github.com/owncloud/core/pull/11305#issuecomment-57150156

Fix for https://github.com/owncloud/core/issues/11008

- [x] Add hooks for add/remove mount point
- [x] Update etag for all applicable users
- [x] Unit tests
Can you add a comment: is `$user` the currently logged in user or the user for who the mount point is applicable ? (which means it can also be `all`). I'm confused between `$this->user` and the applicable user passed in the update hook. A bit for PHP doc might help to clarify :smile: