Length isalive timeout to 10 minutes (from 1 minute) for PLAB nodes

(slivers). Account updates take longer of course, but thats okay for
now.

tmcd/common/watchdog

@@ -41,7 +41,7 @@ my $timeout	= (60 * 60 * 12);	# In seconds of course.
 my $logname	= "$LOGDIR/emulab-watchdog.debug";
 my $pidfile	= "/var/run/emulab-watchdog.pid";
 my $debug	= 0;
-my $isalivewait = ((REMOTE() == 1) ? 60 :
+my $isalivewait = ((REMOTE() == 1) ? (PLAB() ? 600 : 60) ?
 		   (JAILED() ? 600 : 180)); # Seconds to wait.
 my $driftfile;
 my $vnodeid;