pbs: in promise, do not hide stderr output

slapos/recipe/pbs.py

@@ -44,7 +44,7 @@ def promise(args):
   with open("/dev/null") as _dev_null:
     ssh = subprocess.Popen(
         [args['ssh_client'], '%(user)s@%(host)s' % args, '-p', '%(port)s' % args],
-        stdin=subprocess.PIPE, stdout=_dev_null, stderr=_dev_null
+        stdin=subprocess.PIPE, stdout=_dev_null, stderr=None
     )
 
   # Rdiff Backup protocol quit command