[sonic-cfggen] Add Mellanox platform specific code to read base mac from machine.conf

[keboliu]

- What I did
With Mellanox ONIE release(2019.05-5.2.0012) and above, "onie_base_mac" was added to "/host/machine.conf":

onie_base_mac=e4:1d:2d:44:5e:80

So now on Mellanox platform, we have another way to get the mac address besides decode syseeprom, by this can mitigate the dependency on the hw-management service if switch installed latest ONIE.

- How I did it
Add code to get_system_mac() for Mellanox platform to check whether base mac is availaible in "machine.conf", if not, fall back on the old way.

- How to verify it
run "sonic-cfggen -H --print-data" with the new code

admin@mtbc-sonic-03-2700:/tmp# sudo sonic-cfggen -H --print-data
{
    "DEVICE_METADATA": {
        "localhost": {
            "mac": "e4:1d:2d:44:5e:80", 
            "platform": "x86_64-mlnx_msn2700-r0"
        }
    }
}

- Description for the changelog

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

[keboliu]

retest this please

[keboliu]

retest vs please