[generic-config-updater] Add NTP validator (#2212)

What I did
When GCU config NTP_SERVER table, its change is not actually taken info effect. Because NTP service is not restarted. So I add NTP service validator and restart service when NTP_SERVER is changed.

How I did it
When NTP_SERVER table being configured through GCU, the ntp service will restart.

How to verify it
Run GCU E2E test for NTP_SERVER table.

generic_config_updater/generic_config_updater.conf.json

@@ -45,6 +45,9 @@
         },
         "ACL_RULE": {
             "services_to_validate": [ "caclmgrd-service" ]
+        },
+        "NTP_SERVER": {
+            "services_to_validate": [ "ntp-service" ]
         }
     },
     "services": {
@@ -65,6 +68,9 @@
         },
         "caclmgrd-service": {
             "validate_commands": [ "generic_config_updater.services_validator.caclmgrd_validator" ]
+        },
+        "ntp-service": {
+            "validate_commands": [ "generic_config_updater.services_validator.ntp_validator" ]
         }
     }
 }

generic_config_updater/services_validator.py

@@ -98,3 +98,6 @@ def caclmgrd_validator(old_config, upd_config, keys):
     # No update to ACL_RULE.
     return True
 
+
+def ntp_validator(old_config, upd_config, keys):
+    return _service_restart("ntp-config")