RingBuilder._reassign_parts assumed that only replica for a given
partition would move. This isn't necessarily true in the case where a
bunch of devices have been removed. This would leave invalid entries
in _replica2part2dev and also cause validation to fail.
One easy way to reproduce this is to create a 3-replica, 3-zone,
6-device ring with 2 drives per zone (all of equal weight), rebalance,
and then remove one drive from each zone and rebalance again.
Bug: 943493
Change-Id: I0d399bed5d733448ad877fa2823b542777d385a4
7fe0c6c695