Shutdown tweaks, inspired by replication2

A couple of fixes in here.

1) Added some checks that the node is actually down. @buddhisthead already has this in a branch, but like Veruca Salt, I WANT IT NOOOOOOOOOOOOW
2) Removed kill -9 logic from wait_until_unpingable
3) Hard coded a timeout of 6 minutes to wait_until_unpingable, since init:stop could take as long as 5 minutes
4) Added kill -0 monitoring to rtdev:stop to make sure a node is actually stopped before exiting a method.