[201911] Create Vxlan and Vnet default configs

[prsunny]

Why I did it

Create Vxlan and Vnet default configs as below for certain cases:

    "VNET": {
        "Vnet-default": {
            "scope": "default",
            "vni": "8000",
            "vxlan_tunnel": "Tunnel-default"
        }
    },
    "VXLAN_TUNNEL": {
        "Tunnel-default": {
            "src_ip": "10.1.0.32"
        }
    },

How I did it

Master PR - #13409

How to verify it

Which release branch to backport (provide reason below if selected)

• 201811
• 201911
• 202006
• 202012
• 202106
• 202111
• 202205
• 202211

Description for the changelog

Ensure to add label/tag for the feature raised. example - PR#2174 under sonic-utilities repo. where, Generic Config and Update feature has been labelled as GCU.

Link to config_db schema for YANG module changes

A picture of a cute animal (not mandatory but encouraged)

[abdosi]

@prsunny can you created PR for 201911 PR checker failure after this PR. Looks like PR checker were bypassed as part of this PR merge