task/cray configuration ordering bug

CRAY systems only: TaskPlugins must list task/cgroup before task/cray in
    order for the cgroup files to be created before task/cray runs. Without
    this change, the task/cray plugin frequently produces errors about the
    "mems" file being missing. The errors don't seem consistent, so this
    probably involves a race condition. Note that NERSC uses this order
    today and I changed read_config.c to produce a fatal error if the order
    is reversed.