Shutdown or restart pending -button for Coin

Just like Jenkins has, we could use a button in the GUI so that whenever we have a restart coming up, like a maintenance break, we hit the button and it will suspend taking any new jobs, nor will it start performing any jobs in queue. In other words, it will not clone any new VMs, but it will complete the current ones. So after a while, all the hosts will be empty and no jobs have to killed at shutdown.