Fix rebalance for zero weighted devices.

If we set device's weight to zero, currently balance will be set
special value(999.99) until zero weighted device return all
its partitions. So we cannot check balance has changed.
Thus we need to check balance or last_balance is special value.

Change-Id: I5b7db8b8e48db0c4771c51a764bda689869817d5
Fixes: bug #1171731
Merge "Fix rebalance for zero weighted devices."