bb/cray: add support for "equalize_fragments"

burst_buffer/cray - Add support for rounding up the size of a buffer reqeust
    if the DataWarp configuration "equalize_fragments" is used. That option
    can significantly increase the size of a burst buffer allocation in order
    to create equal sized buffers on all server nodes for performance reasons.
    Support current only provided by Cray for job buffers, not persistent
    burst buffers.