If the ring has been rebalanced recently, the next call to rebalance
will not rebalance again if the time passed is shorter than
min_part_time. swift-ring-builder returns an exit code of 1 in this
case, which is a warning, but not a hard error (errors will use an exit
code of 2).
It makes sense therefore to expect an exit code of 0 or 1 and only fail
if it's different.
Closes-Bug: 1677588
Change-Id: I03dd075c590344a920099c57b6a7530f103e0ede
Christian Schwede