Host syntax consists of a comma-delimited list of node names, each
entry optionally containing a ":N" extension indicating the number of
slots to assign to that entry:

   --host node01:5,node02

In the absence of the slot extension, one slot will be assigned to the
node. Duplicate entries are aggregated and the number of slots
assigned to that node are summed together.

Note:

  A "slot" is the PRRTE term for an allocatable unit where we can
  launch a process. Thus, the number of slots equates to the maximum
  number of processes PRRTE may start on that node without
  oversubscribing it.
