Generate interface table to have an entry for default VRF

[prsunny]

- What I did
Generate interface table to have an entry designated to default VRF during config reload. This is when the interface is not configured to be part of any VRFs. This is a placeholder PR to be merged along with sonic-net/sonic-buildimage#2848

Current:

    "VLAN_INTERFACE": {
        "Vlan1000|192.168.0.1/21": {}
    },

Proposed:

    "VLAN_INTERFACE": {
        "Vlan1000": {},
        "Vlan1000|192.168.0.1/21": {}
    },

- How I did it
DB migrator modified to migrate interface table during config reload

- How to verify it
Run config reload and verify for entries without prefix in config_db

 2) "LOOPBACK_INTERFACE|Loopback0|FC00:1::32/128"
 3) "MGMT_INTERFACE|eth0|FC00:2::32/64"
 4) "PORTCHANNEL_INTERFACE|PortChannel0002|FC00::75/126"
 5) "PORTCHANNEL_INTERFACE|PortChannel0001|FC00::71/126"
 6) "PORTCHANNEL_INTERFACE|PortChannel0004|10.0.0.62/31"
 7) "PORTCHANNEL_INTERFACE|PortChannel0001"
 8) "PORTCHANNEL_INTERFACE|PortChannel0004|FC00::7D/126"
 9) "PORTCHANNEL_INTERFACE|PortChannel0003|10.0.0.60/31"
10) "PORTCHANNEL_INTERFACE|PortChannel0003"
11) "PORTCHANNEL_INTERFACE|PortChannel0004"
12) "PORTCHANNEL_INTERFACE|PortChannel0003|FC00::79/126"
13) "VLAN_INTERFACE|Vlan1000|192.168.0.1/21"
14) "LOOPBACK_INTERFACE|Loopback0|10.1.0.32/32"
15) "PORTCHANNEL_INTERFACE|PortChannel0001|10.0.0.56/31"
16) "MGMT_INTERFACE|eth0|10.3.147.47/23"
17) "VLAN_INTERFACE|Vlan1000"
18) "PORTCHANNEL_INTERFACE|PortChannel0002|10.0.0.58/31

- 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)

[prsunny]

retest this please

[prsunny]

retest this please