Human readable hcheck functionality

Full hcheck functionality is implemented in this patch. Hcheck runs
per-group checks, provides summary for the entire cluster and simulates
rebalance if required, the printing the modified metrics.
In this patch, only the human-readable output is available.

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