When NodeMonitor is reconfigured or when the instance goes online, recompute the values.

Based on the following IRC conversations:

 (04:02:29 PM) cliffano: my build slave is offline, "Disk space is too low. Only 0.331GB left on" , but the box has 5.5Gb free.. i've tried disabling the disk space checking, restarted both master and slave, that error message is still there
 (04:03:03 PM) rtyler: I've seen that a couple times, I think I just disconnected and reconnected the slave
 (04:03:47 PM) cliffano: no dice for me.. reconnected several times.. even tried deleting more stuffs to free up more space, still exactly the same error message
 (04:04:04 PM) rtyler: you could try reinstalling windows
 (04:04:06 PM) ***rtyler ducks
 (04:04:22 PM) cliffano: lol yea
 (04:04:49 PM) kohsuke: cliffano: you can force the recomputation of those metrics
 (04:04:56 PM) kohsuke: otherwise it's only once an hour
 (04:05:17 PM) rtyler: wait, you can force it?
 (04:05:37 PM) kohsuke: http://ci.jenkins-ci.org/computer/ "refresh status"
 (04:06:26 PM) rtyler: ah
 (04:07:25 PM) cliffano: kohsuke: thanks, that worked..
 (04:07:43 PM) kohsuke: although I expect that if you've been reconnecting that should be sufficient
 (04:08:08 PM) kohsuke: And I probably should also do that automatically when you reconfigure the column, threashold, etc
 (04:10:39 PM) cliffano: kohsuke: reconnecting didn't help somehow... i tried disconnecting from the UI, tried killing the process on the slave machine, restarted etc
 (04:10:56 PM) kohsuke: it's probably caching the value
 (04:12:16 PM) cliffano: kohsuke: does it cache in the filesystem? i restarted both master and slaves
 (04:13:09 PM) kohsuke: it's stored in Computer
 (04:13:11 PM) kohsuke: so I guess that explains
 (04:13:34 PM) kohsuke: let me clear this when a slave connects
 (04:16:23 PM) cliffano: kohsuke: thanks :)