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
Is `$user` always an existing user or can it be null ?
Still struggling a bit to understand what is the code path for the user "all"