Refactor: scheduler: split can_run_instance() into two functions

can_run_instance() had two completely separate modes of behavior: the original
caller would pass a NULL node which would iterate over all allowed nodes using
recursive calls with a non-NULL node.

There's no need for recursion, just separate it into two functions. It also
makes the naming make more sense, since the function previously returned void
and updated allowed node scores. Now, can_run_instance() is a bool check as
would be expected, and the new ban_unavailable_allowed_nodes() updates the
scores.