hbal: relax restrictions on split instances

The recent patch series on hcheck made me realise how much pain we
have due to hbal not being able to work (at all) with split instances.

This is sub-optimal; ideally, hbal would degrade its behaviour, but
not completely refuse to work. This patch does this, by marking any
split instances as not movable (since such instances most likely mean
that some manual intervention is done on then), but otherwise
proceeding with the rebalance of the selected node group.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>