Print info about ability to rebalance

As suggested during offline discussion, this patch adds a new bash
variable and description field in hcheck, which informs if the
rebalance was possible. This is to determine if there are split
instances on the cluster, in which case rebalance is impossible.

Signed-off-by: Agata Murawska <agatamurawska@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>