Add sub port interface to config interface ip add/del

[wendani]

- What I did
Implement HLD sonic-net/SONiC#420

- How I did it

- How to verify it
On mlnx dut

default mode:

    "VLAN_SUB_INTERFACE": {
        "Ethernet64.10": {
            "admin_status": "up"
        },
        "Ethernet64.10|10.0.0.32/31": {},
        "Ethernet64.10|FC00::41/126": {}
    },

sudo config interface ip remove Ethernet64.10 10.0.0.32/31

    "VLAN_SUB_INTERFACE": {
        "Ethernet64.10": {
            "admin_status": "up"
        },
        "Ethernet64.10|FC00::41/126": {}
    },

sudo config interface ip remove Ethernet64.10 FC00::41/126

VLAN_SUB_INTERFACE table is removed from CONFIG_DB

sudo config interface ip add Ethernet64.10 10.0.0.32/31

    "VLAN_SUB_INTERFACE": {
        "Ethernet64.10": {
            "admin_status": "up"
        },
        "Ethernet64.10|10.0.0.32/31": {}
    },

sudo config interface ip add Ethernet64.10 FC00::41/126

    "VLAN_SUB_INTERFACE": {
        "Ethernet64.10": {
            "admin_status": "up"
        },
        "Ethernet64.10|10.0.0.32/31": {},
        "Ethernet64.10|FC00::41/126": {}
    },

alias mode:

sudo config interface ip remove etp17.10 10.0.0.32/31 

    "VLAN_SUB_INTERFACE": {
            "Ethernet64.10": {
            "admin_status": "up"
        },
        "Ethernet64.10|FC00::41/126": {}
    },

sudo config interface ip remove etp17.10 FC00::41/126

VLAN_SUB_INTERFACE table is removed from CONFIG_DB

sudo config interface ip add etp17.10 10.0.0.32/31  

    "VLAN_SUB_INTERFACE": {
        "Ethernet64.10": {
            "admin_status": "up"
        },
        "Ethernet64.10|10.0.0.32/31": {}
    },

sudo config interface ip add etp17.10 FC00::41/126

    "VLAN_SUB_INTERFACE": {
        "Ethernet64.10": {
            "admin_status": "up"
        },
        "Ethernet64.10|10.0.0.32/31": {},
        "Ethernet64.10|FC00::41/126": {}
    },

- 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]

May be it is good now to just set a table_name here and use config_db.set_entry once instead of repeated lines of same code.

[wendani]

retest this please