[sonic-py-common] Add platform and chassis info methods to device_info

[alexrallen]

Why I did it

These methods were added to make some convenient platform and chassis information methods accessible through sonic-py-common. These methods were refactored from sonic-utilities and are used in the show platform summary and show version commands.

How I did it

There are two methods, one is get_platform_info() which simply calls local methods to collect useful platform information into a dictionary format, this came directly from sonic-utilities.

The other method is get_chassis_info() which retrieves the chassis model, serial number, and hardware revision from the STATE_DB directly. This is also utilized by sonic-utilities which is added by alexrallen/sonic-utilities#3

How to verify it

Open a python3 interpreter and enter the following lines

Python 3.7.3 (default, Jan 22 2021, 20:04:44)
[GCC 8.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from sonic_py_common import device_info
>>> device_info.get_platform_info()
{'platform': 'x86_64-mlnx_msn2700-r0', 'hwsku': 'ACS-MSN2700', 'asic_type': 'mellanox', 'asic_count': 1}
>>> device_info.get_chassis_info()
{'model':'MSN2700-CS2FO', 'serial':'MT1623X09522', 'revision': 'A1'}

For different platforms these values will be different, simply check if they are successfully populated. If alexrallen/sonic-platform-daemons#3 has not been merged then get_chassis_info() is expected to return an empty dictionary.

Description for the changelog

[sonic-py-common] Add platform and chassis info methods to device_info

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

[Junchao-Mellanox]

It looks good to me, but could you please add unit test case for your change?

[alexrallen]

@keboliu @Junchao-Mellanox I have updated the unit test for the existing show platform summary test.