[202012][minigraph] Consume golden_config_db.json while loading minigraph

[wen587]

What I did

This PR is to cherry-pick changes in PR #2140 to 202012 branch after resolving conflicts.
This PR is for supporting consume golden_config_db.json while loading minigraph. User can put the golden_config_db.json with minigraph file to override configDB after reload minigraph.
The golden_config_db.json looks just like a config_db.json and it will be placed on /etc/sonic/golden_config_db.json.
Step1: Load minigraph to configDB
Step2: If golden_config_db.json exists and contains some Table configuration, that Table in configDB will be overriden by the config in golden_config_db.json. Other config that not included in golden_config_db.json will keep the same as minigraph.
For example, assume below ACL_TABLE is loaded from minigraph:
config from minigraph:

        "ACL_TABLE": {
            "DATAACL": {
                "policy_desc": "DATAACL",
                "ports": [
                    "Ethernet4"
                ],
                "stage": "ingress",
                "type": "L3"
            },
            "NTP_ACL": {
                "policy_desc": "NTP_ACL",
                "services": [
                    "NTP"
                ],
                "stage": "ingress",
                "type": "CTRLPLANE"
            }
        },

The golden_config_db.json also contains ACL_TABLE as below.
config from golden_config_db.json

"ACL_TABLE": {
            "EVERFLOWV6": {
                "policy_desc": "EVERFLOWV6",
                "ports": [
                    "Ethernet12"
                ],
                "stage": "ingress",
                "type": "MIRRORV6"
            }
        },

During load_minigraph, the final config will be the same with golden_config_db.json because ACL_TABLE will be overriden by golden_config_db.json.
configDB ACL_TABLE final state:

"ACL_TABLE": {
            "EVERFLOWV6": {
                "policy_desc": "EVERFLOWV6",
                "ports": [
                    "Ethernet12"
                ],
                "stage": "ingress",
                "type": "MIRRORV6"
            }
        },

How I did it

Add code config override-config-table command to let it consume golden_config_db.json

How to verify it

Add UT tests and run.

Previous command output (if the output of a command-line utility has changed)

New command output (if the output of a command-line utility has changed)

admin@vlab-01:~$ sudo config override-config-table -h
Usage: config override-config-table [OPTIONS] INPUT_CONFIG_DB

  Override current configDB with input config.

Options:
  --dry_run TEXT  Dry run, writes config to the given file
  -h, -?, --help  Show this message and exit.