[GCU] apply-patch failed to get token from 'using' group

[wen587]

Description

When apply-patch a json file which yang has grouping member, the method cannot grab token.

Steps to reproduce the issue

1. bgp-monitor.yang has member that defined in sonic-bgp-common.yang
   https://github.com/Azure/sonic-buildimage/blob/master/src/sonic-yang-models/yang-models/sonic-bgp-monitor.yang#L43
2. When apply-patch, it failed to grab token.

admin@vlab-01:~$ sudo config apply-patch -i /FEATURE -i /QUEUE -i /SCHEDULER bgpl/add.json
Patch Applier: Patch application starting.
Patch Applier: Patch: [{"op": "add", "path": "/BGP_MONITORS", "value": {"11.0.0.1": {"admin_status": "up", "asn": "65100", "holdtime": "10", "keepalive": "3", "local_addr": "10.1.0.32", "name": "BGPMonitor", "nhopself": "0", "rrclient": "0"}}}]
...
Failed to apply patch
Usage: config apply-patch [OPTIONS] PATCH_FILE_PATH
Try "config apply-patch -h" for help.

Error: Token not found 

3. some debug info when error occur in https://github.com/Azure/sonic-utilities/blob/master/generic_config_updater/gu_common.py#L551
   raise ValueError("Token not found model {} token_index {} path_tokens {}".format(model, token_index, path_tokens))
   The token is only got from leaf not from using

model OrderedDict([('@name', 'BGP_MONITORS_LIST'), 
('key', OrderedDict([('@value', 'addr')])), 
('description', OrderedDict([('text', 'This list is to support BGP monitor configuration handled by bgpcfgd')])), 
('leaf', OrderedDict([('@name', 'addr'), ('type', OrderedDict([('@name', 'inet:ip-address')])), 
('description', OrderedDict([('text', 'BGP monitor address')])), ('__isleafList', False)])), 

('uses', OrderedDict([('@name', 'bgpcmn:sonic-bgp-cmn-neigh'),    <===================================================

('refine', OrderedDict([('@target-node', 'name'), 
('must', OrderedDict([('@condition', "current() = 'BGPMonitor'"), 
('error-message', OrderedDict([('value', 'Invalid BGP monitor name')]))]))]))]))]) 
token_index 2 path_tokens ['BGP_MONITORS', '11.0.0.1', 'asn']

Describe the results you received

See above.

Describe the results you expected

Fix in two ways:

1. We don't using grouping in Yang such that bgp-monitor has all member defined in itself.
2. Handle using keyword in yang model.

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

Output of show version

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