If a device with 0 weight is tried to remove, the following rebalance
does not write changes into builder file.
Scenario:
$ swift-ring-builder object.builder set_weight --id 1 0.00
$ swift-ring-builder object.builder rebalance
Wait for moving files out of the device id=1.
$ swift-ring-builder object.builder remove --id 1
$ swift-ring-builder object.builder rebalance
In fact, the device id=1 is not removed after rebalance (must be --force used).
Change-Id: Iad5a444023eae9882a3addd7f119ff4d18559ddd