Pack step support changes

Send pack job's task IDs, node count, tasks_per_node and node_list
  from srun to slurmd as needed by MPI. This information is constructed
  by srun combining information from each individual job component's
  step information.
Set global SLURM_NODELIST, SLURM_JOB_NODELIST, SLURM_STEP_NUM_NODES,
  SLURM_TASKS_PER_NODE, SLURM_NNODES, SLURM_NTAKS and SLURM_STEP_NUM_TASKS
   for pack step.
Modify pmi2 plugin to launch a single agent for pack step rather than
   one per component.
Modify pmi2 and pmix plugins to gather global step information for
   pack steps.
Make task distribtion be configurable by step component (i.e. "block"
   on one and "cyclic" on another).