pmp-check-mongo: fixed check_balance

[jkramarz]

Shards' names has small chance to be properly balanced, using number of chunks in shard may be a better option ;-)

>>> for i in {u'bdd4b386': 5, u'ba142fa9': 5, u'5904c137': 4}:
...   print(i)
... 
bdd4b386
ba142fa9
5904c137