modify task layout with --overcommit

Modify task layout with --overcommit option plus a heterogeneous job
    allocation so that a cyclic task distribution can start happening before
    all CPUs on all nodes are fully allocated. The number of tasks per node
    will be unchanged from the previous algorithm, but tasks will be distributed
    in a cyclic fashion first and then extra tasks placed on nodes with more
    CPUs. Previously all CPUs would be fully allocated in a cyclic fashion,
    then excess tasks distributed evenly across all allocated nodes.
Bug 6981