Add constants for a ping confd request

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>

lib/constants.py

@@ -625,10 +625,12 @@ MASTER_POOL_SIZE_DEFAULT = 10
 
 CONFD_PROTOCOL_VERSION = 1
 
+CONFD_REQ_PING = 0
 CONFD_REQ_NODE_ROLE_BYNAME = 1
 CONFD_REQ_NODE_PIP_BY_INSTANCE_IP = 2
 
 CONFD_REQS = frozenset([
+  CONFD_REQ_PING,
   CONFD_REQ_NODE_ROLE_BYNAME,
   CONFD_REQ_NODE_PIP_BY_INSTANCE_IP,
   ])