Fix broken SQL to identify trap agent

All netboxes have an IP address, but not all netboxes have
interfaces or even gwports, so left joins are necessary to still have
matches.

python/nav/snmptrapd/trap.py

@@ -66,9 +66,9 @@ def _lookup_agent(self):
         cur = conn.cursor()
         cur.execute(
             "SELECT DISTINCT netboxid, sysname, roomid "
-            "FROM gwportprefix "
-            "JOIN interface USING (interfaceid) "
-            "JOIN netbox USING (netboxid) "
+            "FROM netbox "
+            "LEFT JOIN interface USING (netboxid) "
+            "LEFT JOIN gwportprefix USING (interfaceid) "
             "WHERE %s IN (ip, gwip) ",
             (self.agent,),
         )