[yang] Update device_metadata to add dhcp_server (#10598)

Why I did it
dhcp_server is introduced, and need to update yang model.

How I did it
Update yang models and add unit test.

How to verify it
Run unit test for sonic-yang-models.

Signed-off-by: Gang Lv ganglv@microsoft.com

src/sonic-yang-models/tests/files/sample_config_db.json

@@ -283,7 +283,8 @@
                 "hwsku": "Stone",
                 "buffer_model": "dynamic",
                 "cloudtype": "Public",
-                "region": "usfoo"
+                "region": "usfoo",
+                "dhcp_server": "disabled"
             }
         },
         "VLAN": {

src/sonic-yang-models/tests/yang_model_tests/tests/device_metadata.json

@@ -93,6 +93,13 @@
 	"DEVICE_METADATA_INVALID_STORAGE_DEVICE": {
 		"desc": "Verifying invalid storage device value",
 		"eStrKey": "InvalidValue"
+    },
+    "DEVICE_METADATA_VALID_DHCP_SERVER": {
+        "desc": "Verifying dhcp_server configuration."
+    },
+    "DEVICE_METADATA_INVALID_DHCP_SERVER": {
+        "desc": "Verifying invalid dhcp_server configuration.",
+        "eStrKey": "InvalidValue"
     }
 
 }

src/sonic-yang-models/tests/yang_model_tests/tests_config/device_metadata.json

@@ -251,5 +251,23 @@
                 }
             }
         }
+    },
+    "DEVICE_METADATA_VALID_DHCP_SERVER": {
+        "sonic-device_metadata:sonic-device_metadata": {
+            "sonic-device_metadata:DEVICE_METADATA": {
+                "sonic-device_metadata:localhost": {
+                    "dhcp_server": "enabled"
+                }
+            }
+        }
+    },
+    "DEVICE_METADATA_INVALID_DHCP_SERVER": {
+        "sonic-device_metadata:sonic-device_metadata": {
+            "sonic-device_metadata:DEVICE_METADATA": {
+                "sonic-device_metadata:localhost": {
+                    "dhcp_server": "invalid"
+                }
+            }
+        }
     }
 }

src/sonic-yang-models/yang-models/sonic-device_metadata.yang

@@ -148,6 +148,11 @@ module sonic-device_metadata {
                 leaf storage_device {
                     type boolean;
                 }
+
+                leaf dhcp_server {
+                    type stypes:admin_mode;
+                    description "Indicate whether enable the embedded DHCP server.";
+                }
             }
             /* end of container localhost */
         }