fix alps plm to allow explicit host placement

    It turns out that the alps plm code was developed only
    on cray systems that were running batch schedulers.

    However, for bring up and development systems, its not
    at all uncommon for there to be no batch scheduler, and
    thus to orte it appears that orte_num_allocated_nodes
    is always zero.  This forces a user using mpirun on such
    a system to always specify a host list:

    mpirun -n 4 -N 1 -host 32,45,68 ....

    just to get the job to run, but then since the -L argument for aprun
    is never built, the app always runs on the first batch of nodes that
    aprun finds available.