[Mellanox] Add support for set/get system led status

[Junchao-Mellanox]

- Why I did it

System health feature needs to set/get system led status

- How I did it

1. Add a led object in chassis class and initialize it when the API is called on host side
2. Read/write system led system fs to get/set the status

- How to verify it

Manual test on SN2700

- Description for the changelog

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

[Junchao-Mellanox]

Retest vsimage please

[sujinmkang]

@Junchao-Mellanox Can we just read "/var/run/hw-management/led/led_status" for the get_status()?

[Junchao-Mellanox]

@Junchao-Mellanox Can we just read "/var/run/hw-management/led/led_status" for the get_status()?

No, we cannot for now because driver doesn't put the LED status to that file as I tested.

[Junchao-Mellanox]

Retest vsimage please.

[keboliu]

retest vsimage please