Fix rebalance crash post-2.1.0 upgrade.

If you had a builder file from an earlier version of Swift (before
6d77c37) and you attempted to rebalance it with a new version of Swift
(after 6d77c37, before this commit), you could get a crash.

Commit 6d77c37 changed the calculation for parts_wanted slightly
(round up instead of round down), so if a rebalance is attempted
without recomputing parts_wanted, then swift-ring-builder crashes.

The crash only occurs if the first command run is "rebalance". Adding
a device, removing a device, changing a device's weight, or changing
replica count (basically, anything that gives you a reason to
rebalance) will all cause parts_wanted to be recalculated, so
subsequent rebalances will succeed.

Change-Id: I6f7a69701f74546a00d3183aa7763e9e708408c9