core: remove snapshot - validate snapshot type

* Validate snapshot type on RemoveSnapshotCommand
  (e.g. Active snapshots cannot be removed).
* Removed the obsolete validation based on a representative image.
* Added appropriate tests accordingly.

Change-Id: I90d9144e9d0f7db4f165b7d6945bb27d387c4f66
Bug-Url: https://bugzilla.redhat.com/1140207
Signed-off-by: Daniel Erez <derez@redhat.com>