Added SchedulerParameters option "bf_busy_nodes"

Added SchedulerParameters option of "bf_busy_nodes". When selecting
resources for pending jobs to reserve for future execution (i.e. the job
can not be started immediately), then preferentially select nodes that are
in use. This will tend to leave currently idle resources available for
backfilling longer running jobs, but may result in allocations having less
than optimal network topology. This option is currently only supported by
the select/cons_res plugin.