Allow rebalance to take a seed.

Passing a seed into rebalance makes the rebalance deterministic
which allows us to generate identical rings across disparate
nodes without having to copy the ring files around.

Change-Id: Ie5ae46ac030e61284bc501fdef9d77eeb5243afd
Cleanup based on pyflakes.

pyflakes itself can't be used in any automated gating way, because there are
two sets of false errors it raises. However, as an exercise, cleaning up the
'valid' ones uncovered three actual bugs. The other changes (mostly unused
variables) are included here for fun.

Command run: pyflakes swift | grep -v "undefined name '_'"

Change-Id: I18696bf047dedad1a9fdbde3463e214fba95f7c6
Merge "Cleanup based on pyflakes."
Merge "Allow rebalance to take a seed."