Let some swift-ring-builder commands take >1 arg.

`swift-ring-builder X.builder [add|set_weight|set_info|remove]` can
now work on multiple devices at once.

This allows for faster bulk operations; for example, if an
administrator has to add 48 new devices to the ring, then this lets
them do it in one command invocation. Since each invocation of
swift-ring-builder follows a deserialize-operate-serialize pattern,
using the bulk form of the commands on N devices will save N-1
[de]serializations.

Change-Id: Ibf77528f28026866af3c94d0370181c8013d5bc5

Author: smerritt