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 quick doc about how this propagator works and mention the two modes of operation ? (user and dirty flag)