Reset `group_id` during cluster deletion in FakeUI

If we add some node to some cluster and then remove this cluster, we
won't be able to add this node to some another cluster. The cause is
that we have a "hack" that handles cluster removing in Fake UI mode.

Change-Id: Ib2981ceb9769455f0c3fa2e0b0de10b5f083eeef
Closes-Bug: #1398822
Merge "Reset `group_id` during cluster deletion in FakeUI"