[lldpmgrd] Fix lldpmgrd issue where it sets the lldp system name to None

- Only check hostname field in key DEVICE_METADATA|localhost of configDB
- If hostname does not exist, don't set lldp system name

Signed-off-by: Zhenggen Xu <zxu@linkedin.com>

dockers/docker-lldp/lldpmgrd

@@ -245,10 +245,13 @@ class LldpManager(daemon_base.DaemonBase):
         if not op in ["SET", "DEL"]:
             return
         self.log_info("Device Config Opcode: {} Dict {} Key {}".format(op, device_dict, key))
-        hostname = device_dict.get("hostname")
-        if not self.hostname == hostname:
-            self.log_info("Hostname changed old {0}, new {1}".format(self.hostname, hostname))
-            self.update_hostname(hostname)
+        # only check hostname in key:localhost
+        if key == "localhost":
+            hostname = device_dict.get("hostname")
+            # check hostname is valid and not the same as before
+            if hostname and not self.hostname == hostname:
+                self.log_info("Hostname changed old {0}, new {1}".format(self.hostname, hostname))
+                self.update_hostname(hostname)
 
     def lldp_process_port_table_event(self, key, op, fvp):
         if (key != "PortInitDone") and (key != "PortConfigDone"):