Add min_tasks_per node schedule parameter

[tygoetsch]

This would allow the following functionality in a test:

base:
  schedule:
    tasks_per_node: "{{sched.min_tasks_per}}"

[Paul-Ferrell]

Not quite what I was going for. That still has the problem of using a variable to produced by scheduling to configure scheduling.

More like:

base:
  schedule:
    tasks_per_node: all
    min_tasks_per_node: 32