[FLINK-17336][CI] Update watchdog to properly kill watched process

As part of FLINK-16411, we introduced a run_mvn bash function. A side
effect of this is that the watchdog was killing the run_mvn function
call, but not the child process of that function (in this case)
Maven.
With this change, we are killing the function and all children
of that function.

Reference: https://stackoverflow.com/questions/2618403/how-to-kill-all-subprocesses-of-shell

tools/travis_watchdog.sh

@@ -222,7 +222,8 @@ watchdog () {
 
 			print_stacktraces | tee $TRACE_OUT
 
-			kill $(<$CMD_PID)
+			# Kill $CMD and all descendants
+			pkill -P $(<$CMD_PID)
 
 			exit 1
 		fi