[yang-models] device_metadata validation issue

[ganglyu]

Description

'DEVICE_METADATA': {'localhost': {'bgp_asn': '65100',
'cloudtype': 'Public',
'deployment_id': '1',
'docker_routing_config_mode': 'separated',
'downstream_subrole': 'downstream_subrole_y',
'hostname': 'switch-t0',
'hwsku': 'Force10-S6000',
'region': 'usfoo',
'resource_type': 'resource_type_x',
'synchronous_mode': 'enable',
'type': 'ToRRouter'}},

Config db schema generated by minigraph can’t pass yang validation:

• there's no downstream_subrole field in sonic-device_metadata.yang

It seems that we need to update sonic-config-engine or yang model.

Steps to reproduce the issue:

1. Run command 'sonic-cfggen -m tests/simple-sample-graph-metadata.xml --print-data'.
2. Use sonic_yang to validate config db schema.

Describe the results you received:

Yang validation failed.

Describe the results you expected:

Yang validation should pass.

Output of show version:

(paste your output here)

Output of show techsupport:

(paste your output here or download and attach the file here )

Additional information you deem important (e.g. issue happens only occasionally):

[ganglyu]

@anish-n
https://github.com/Azure/sonic-buildimage/blob/master/src/sonic-config-engine/tests/simple-sample-graph-metadata.xml#L261-L265
If downstream_subrole is necessary, could you please update sonic-device_metadata.yang?