Don't allocate downed cloud nodes

which were marked down due to ResumeTimeout.

If a cloud node was marked down due to not responding by ResumeTimeout,
the code inadvertently added the node back to the avail_node_bitmap --
after being cleared by set_node_down_ptr(). The scheduler would then
attempt to allocate the node again, which would cause a loop of hitting
ResumeTimeout and allocating the downed node again.

Bug 5264