[config reload] Fixing config reload when timer based delayed services are disabled (#1967)

#### What I did
When timer based delayed services like mgmt-framework, telemetry and snmp are disabled and config reload is execute it fails Failed to reset failed state of unit mgmt-framework.service: Unit mgmt-framework.service not loaded.
The reason is these services don't get masked like regular services and these are derived from timers. So when reset-failed is tried on these services it leads to exception.


#### How I did it
When the feature related to these services are disabled their timers would be masked and wouldn't be "enabled". So when deriving the services from timers the services which are not enabled will be skipped.


#### How to verify it
Disable services like mgmt-framework, snmp and telemetry and execute config reload. The config reload should execute without failure

Author: dgsudharsan

config/main.py

@@ -732,8 +732,15 @@ def _get_sonic_services():
 
 
 def _get_delayed_sonic_services():
-    out = clicommon.run_command("systemctl list-dependencies --plain sonic-delayed.target | sed '1d'", return_cmd=True)
-    return (unit.strip().rstrip('.timer') for unit in out.splitlines())
+    rc1 = clicommon.run_command("systemctl list-dependencies --plain sonic-delayed.target | sed '1d'", return_cmd=True)
+    rc2 = clicommon.run_command("systemctl is-enabled {}".format(rc1.replace("\n", " ")), return_cmd=True)
+    timer = [line.strip() for line in rc1.splitlines()]
+    state = [line.strip() for line in rc2.splitlines()]
+    services = []
+    for unit in timer:
+        if state[timer.index(unit)] == "enabled":
+            services.append(unit.rstrip(".timer"))
+    return services
 
 
 def _reset_failed_services():

tests/config_test.py

@@ -84,6 +84,8 @@ def mock_run_command_side_effect(*args, **kwargs):
             return 'snmp.timer'
         elif command == "systemctl list-dependencies --plain sonic.target | sed '1d'":
             return 'swss'
+        elif command == "systemctl is-enabled snmp.timer":
+            return 'enabled'
         else:
             return ''
 
@@ -164,7 +166,7 @@ def test_load_minigraph(self, get_cmd_module, setup_single_broadcom_asic):
             mock_run_command.assert_any_call('systemctl reset-failed swss')
             # Verify "systemctl reset-failed" is called for services under sonic-delayed.target 
             mock_run_command.assert_any_call('systemctl reset-failed snmp')
-            assert mock_run_command.call_count == 10
+            assert mock_run_command.call_count == 11
 
     def test_load_minigraph_with_port_config_bad_format(self, get_cmd_module, setup_single_broadcom_asic):
         with mock.patch(