Use terminate() instead of kill() to stop the promise.

This send SIGTERM instead of SIGKILL to the promise. This allow the
promise to catch te signal and do some clean up before exiting.

slapos/grid/slapgrid.py

@@ -465,7 +465,7 @@ class Slapgrid(object):
 
 
         if process_handler.poll() is None:
-          process_handler.kill()
+          process_handler.terminate()
           raise Slapgrid.PromiseError("The promise %r timed out" % promise)
         elif process_handler.poll() != 0:
           stderr = process_handler.communicate()[1]