Fix: scheduler: don't schedule probes of unmanaged resources on pending nodes

Previously, custom_action() would set an action's optional or runnable flag in
the same, exclusive if-else sequence. This means that if an action should be
optional *and* runnable, only one would be set. In particular, this meant that
if a resource is unmanaged *and* its allocated node is pending, any probe would
be set to optional, but not unrunnable, and the controller could wrongly
attempt the probe before the join completed.

Now, optional is checked separately.