[yang] Update device_metadata to add dhcp_server (#10382)

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

@@ -304,7 +304,8 @@
                 "switch_id": "2",
                 "switch_type": "voq",
                 "max_cores": "8",
-                "sub_role": "FrondEnd"
+                "sub_role": "FrondEnd",
+                "dhcp_server": "disabled"
             }
         },
         "VLAN": {

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

@@ -103,6 +103,13 @@
     },
     "DEVICE_METADATA_VALID_SUB_ROLE_CONFIG": {
         "desc": "Verifying valid sub_role configuration."
+    },
+    "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

@@ -283,5 +283,23 @@
                 }
             }
         }
+    },
+    "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

@@ -183,6 +183,11 @@ module sonic-device_metadata {
                     type uint8;
                     description "Maximum number of cores in a VoQ Switch (chassis).";
                 }
+
+                leaf dhcp_server {
+                    type stypes:admin_mode;
+                    description "Indicate whether enable the embedded DHCP server.";
+                }
             }
             /* end of container localhost */
         }