Fix unbalanced rings after initial creation.

Sometimes you'd make a ring that should have a good balance (~= 1),
but it would come out with a poor balance (>= 90). Rebalancing a
couple times would fix it, but it shouldn't be necessary to do that.

This was due to a bug in the ring builder's rebalance() method where,
when placing a replica for a partition, it would pick *one of* the
tiers with the lowest number of replicas, but the choice was made
without regard to parts_wanted.  Now rebalancing pays attention to the
drives' parts_wanted attributes, so the tier with the emptiest drive
gets the replica.

(Also fixed a typo in a completely unrelated comment.)

Change-Id: I1821f15170f0ed212b0ad7c81b97031540ab4460
Merge "Fix unbalanced rings after initial creation."