libyang3 bug: re-order DEVICE_METADATA leaf

For some reason after upgrading to libyang3, tests started throwing
these errors:
```
------------------- Test 247: DEVICE_METADATA DEFAULT VALUE FOR BGP_STATUS FIELD.---------------------
INFO     YANG-TEST:test_yang_model.py:193 Verify xpath: /sonic-device_metadata:sonic-device_metadata/DEVICE_METADATA/localhost/hostname
INFO     YANG-TEST:test_yang_model.py:197 Verify dnode: /sonic-device_metadata:sonic-device_metadata/DEVICE_METADATA/localhost/hostname
INFO     YANG-TEST:test_yang_model.py:200 Verify path value sonic-device_metadata:default_bgp_status is up in {'sonic-device_metadata:hostname': 'DUT-ASW', 'sonic-device_metadata:platform': 'Stone-DX010', 'sonic-device_metadata:default_pfcwd_status': 'disable', 'sonic-device_metadata:bgp_asn': 65000, 'sonic-device_metadata:frr_mgmt_framework_config': False, 'sonic-device_metadata:synchronous_mode': 'enable', 'sonic-device_metadata:yang_config_validation': 'disable', 'sonic-device_metadata:suppress-fib-pending': 'disabled', 'sonic-device_metadata:timezone': 'UTC', 'sonic-device_metadata:nexthop_group': 'disabled'}
ERROR    YANG-TEST:test_yang_model.py:29  Exception >assert 'sonic-device_metadata:default_bgp_status' in {'sonic-device_metadata:bgp_asn': 65000, 'sonic-device_metadata:default_pfcwd_status': 'disable', 'sonic-device_metadata:frr_mgmt_framework_config': False, 'sonic-device_metadata:hostname': 'DUT-ASW', ...}< in /sonic/src/sonic-yang-models/tests/yang_model_tests/test_yang_model.py:201
ERROR    YANG-TEST:test_yang_model.py:29  Exception >DEVICE_METADATA_DEFAULT_BGP_STATUS: Mismatch ['verified'] and < in /sonic/src/sonic-yang-models/tests/yang_model_tests/test_yang_model.py:234
INFO     YANG-TEST:test_yang_model.py:238 DEVICE_METADATA DEFAULT VALUE FOR BGP_STATUS FIELD. Failed

INFO     YANG-TEST:test_yang_model.py:159
------------------- Test 248: DEVICE_METADATA DEFAULT VALUE FOR DOCKER_ROUTING_CONFIG_MODE FIELD.---------------------
INFO     YANG-TEST:test_yang_model.py:193 Verify xpath: /sonic-device_metadata:sonic-device_metadata/DEVICE_METADATA/localhost/hostname
INFO     YANG-TEST:test_yang_model.py:197 Verify dnode: /sonic-device_metadata:sonic-device_metadata/DEVICE_METADATA/localhost/hostname
INFO     YANG-TEST:test_yang_model.py:200 Verify path value sonic-device_metadata:docker_routing_config_mode is unified in {'sonic-device_metadata:hostname': 'DUT-CSW', 'sonic-device_metadata:platform': 'Stone-DX010', 'sonic-device_metadata:default_pfcwd_status': 'disable', 'sonic-device_metadata:bgp_asn': 65001, 'sonic-device_metadata:frr_mgmt_framework_config': False, 'sonic-device_metadata:synchronous_mode': 'enable', 'sonic-device_metadata:yang_config_validation': 'disable', 'sonic-device_metadata:suppress-fib-pending': 'disabled', 'sonic-device_metadata:timezone': 'UTC', 'sonic-device_metadata:nexthop_group': 'disabled'}
ERROR    YANG-TEST:test_yang_model.py:29  Exception >assert 'sonic-device_metadata:docker_routing_config_mode' in {'sonic-device_metadata:bgp_asn': 65001, 'sonic-device_metadata:default_pfcwd_status': 'disable', 'sonic-device_metadata:frr_mgmt_framework_config': False, 'sonic-device_metadata:hostname': 'DUT-CSW', ...}< in /sonic/src/sonic-yang-models/tests/yang_model_tests/test_yang_model.py:201
ERROR    YANG-TEST:test_yang_model.py:29  Exception >DEVICE_METADATA_DEFAULT_DOCKER_ROUTING_CONFIG_MODE: Mismatch ['verified'] and < in /sonic/src/sonic-yang-models/tests/yang_model_tests/test_yang_model.py:234
INFO     YANG-TEST:test_yang_model.py:238 DEVICE_METADATA DEFAULT VALUE FOR DOCKER_ROUTING_CONFIG_MODE FIELD. Failed

INFO     YANG-TEST:test_yang_model.py:159
```

The "fix" is to simply mov those 2 leaf nodes lower in the schema and it
fixes the issue.

This probably needs to be reported to libyang as the schema appears
valid.

Author: bradh352

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

@@ -34,7 +34,7 @@ module sonic-device_metadata {
 
             description "DEVICE_METADATA part of config_db.json";
 
-            container localhost{
+            container localhost {
 
                 leaf hwsku {
                     type stypes:hwsku;
@@ -47,6 +47,20 @@ module sonic-device_metadata {
                     description "asic_id is unique identifier of the asic used by SAI for initialization.";
                 }
 
+                leaf hostname {
+                    type stypes:hostname;
+                }
+
+                leaf platform {
+                    type string {
+                        length 1..255;
+                    }
+                }
+
+                leaf mac {
+                    type yang:mac-address;
+                }
+
                 leaf default_bgp_status {
                     type enumeration {
                         enum up;
@@ -67,20 +81,6 @@ module sonic-device_metadata {
                     default "unified";
                 }
 
-                leaf hostname {
-                    type stypes:hostname;
-                }
-
-                leaf platform {
-                    type string {
-                        length 1..255;
-                    }
-                }
-
-                leaf mac {
-                    type yang:mac-address;
-                }
-
                 leaf default_pfcwd_status {
                     type enumeration {
                         enum disable;