Parse backend switches to vlan sub port interface

[wendani]

- What I did

- How I did it

- How to verify it
On brcm dut

    "VLAN_SUB_INTERFACE": {
        "Ethernet88.10": {
            "admin_status": "up"
        },
        "Ethernet92.10": {
            "admin_status": "up"
        },
        "Ethernet96.10": {
            "admin_status": "up"
        },
        "Ethernet100.10": {
            "admin_status": "up"
        },
        "Ethernet104.10": {
            "admin_status": "up"
        },
        "Ethernet108.10": {
            "admin_status": "up"
        },
        "Ethernet112.10": {
            "admin_status": "up"
        },
        "Ethernet116.10": {
            "admin_status": "up"
        },
        "Ethernet120.10": {
            "admin_status": "up"
        },
        "Ethernet124.10": {
            "admin_status": "up"
        },
        "Ethernet88.10|172.16.8.13/31": {},
        "Ethernet92.10|172.16.8.141/31": {},
        "Ethernet96.10|172.16.9.13/31": {},
        "Ethernet100.10|172.16.9.141/31": {},
        "Ethernet104.10|172.16.10.13/31": {},
        "Ethernet108.10|172.16.10.141/31": {},
        "Ethernet112.10|172.16.11.13/31": {},
        "Ethernet116.10|172.16.11.141/31": {},
        "Ethernet120.10|172.16.12.13/31": {},
        "Ethernet124.10|172.16.12.141/31": {}
    },


        "Ethernet88": {
            "index": "27", 
            "lanes": "121,122,123,124", 
            "description": "SYD03-0102-0401-01BT1:Ethernet8/1", 
            "pfc_asym": "off", 
            "mtu": "9100", 
            "alias": "Ethernet27/1", 
            "admin_status": "up", 
            "speed": "40000"
        }, 
        "Ethernet92": {
            "index": "28", 
            "lanes": "125,126,127,128", 
            "description": "SYD03-0102-0401-02BT1:Ethernet8/1", 
            "pfc_asym": "off", 
            "mtu": "9100", 
            "alias": "Ethernet28/1", 
            "admin_status": "up", 
            "speed": "40000"
        }, 

        "Ethernet120": {
            "index": "35", 
            "lanes": "1,2,3,4", 
            "description": "SYD03-0102-0401-09BT1:Ethernet8/1", 
            "pfc_asym": "off", 
            "mtu": "9100", 
            "alias": "Ethernet35", 
            "admin_status": "up", 
            "speed": "40000"
        }, 
        "Ethernet124": {
            "index": "36", 
            "lanes": "5,6,7,8", 
            "description": "SYD03-0102-0401-10BT1:Ethernet8/1", 
            "pfc_asym": "off", 
            "mtu": "9100", 
            "alias": "Ethernet36", 
            "admin_status": "up", 
            "speed": "40000"
        }

In sonic-cfggen unit test:

$ pytest test_cfggen.py::TestCfgGen::test_minigraph_sub_port_interfaces   
====================================================================== test session starts ======================================================================
platform linux2 -- Python 2.7.12, pytest-3.3.0, py-1.7.0, pluggy-0.6.0
rootdir: /home/localadmin/190607/sonic-buildimage/src/sonic-config-engine, inifile:
collected 1 item                                                                                                                                                

test_cfggen.py .                                                                                                                                          [100%]

=================================================================== 1 passed in 3.61 seconds ====================================================================

    "VLAN_SUB_INTERFACE": {
        "Ethernet0.10": {
            "admin_status": "up"
        }, 
        "PortChannel01.10": {
            "admin_status": "up"
        }, 
        "Ethernet0.10|10.0.0.58/31": {}, 
        "Ethernet0.10|FC00::75/126": {}, 
        "PortChannel01.10|10.0.0.56/31": {}, 
        "PortChannel01.10|FC00::71/126": {}
    }

- Description for the changelog

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

[lguohan]

please add unit test in sonic-cfggen accordingly.

[lguohan]

add unit test in sonic-cfggen

[wendani]

Added #3413 (review)

[lguohan]

can you consoliate the sub interface transformation in one place. once you caculate the phy interface and port channel, then change them to sub interfaces.

[wendani]

Done #3413 (comment)

[wendani]

retest vs please