zebra: evpn l3vni map to vrf nb changes

zebra: evpn l3vni map to vrf nb changes

The set of northbound changes for l3vni configuration
command under vrf.

vrf x
 vni 1000 prefix-routes-only

{
  "frr-vrf:lib": {
    "vrf": [
      {
        "name": "vrf1",
        "frr-zebra:zebra": {
          "l3vni-id": 4001
        }
      },
      {
        "name": "vrf2",
        "frr-zebra:zebra": {
          "l3vni-id": 4002,
          "prefix-only": true
        }
      }
    ]
  }
}

Signed-off-by: Chirag Shah <chirag@cumulusnetworks.com>