(agentxmpp) Fix restart of the agent when the subscribe subsitute is

turned ON/OFF/ON

When the subscribe subsitute is turn off, the machine is not present
anymore in XMPP.

When it turns ON again, the machine is notified. It know that a
reinscription is needed

(cherry picked from commit 8236ab8)

pulse_xmpp_agent/agentxmpp.py

@@ -1326,6 +1326,9 @@ def check_subscribe(self):
                 "Subscription [%s] status = %s"
                 % (self.sub_subscribe, self.presencectrlsubscribe)
             )
+        else:
+            self.presencectrlsubscribe = "unavailable"
+            self.update_plugin()
 
     def stabilized_start(self):
         """