[sonic-py-common] Added an API to get file path containing SONiC version

[smaheshm]

- Why I did it

Added an API to get the file path containing SONiC version so that the API can be mocked during unit tests.

- How I did it

Added the API.

- How to verify it

Manual verification.

Last login: Mon Dec 28 18:32:34 2020 from 100.127.20.23
admin@str-s6000-acs-9:~$ python3
Python 3.7.3 (default, Jul 25 2020, 13:03:44) 
[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_sonic_version_file()
'/etc/sonic/sonic_version.yml'
>>> 

- Which release branch to backport (provide reason below if selected)

• 201811
• 201911
• 202006

- Description for the changelog

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

[jleveque]

The new get_sonic_version_file() function isn't called anywhere. How do we benefit from mocking it in unit tests?

[smaheshm]

The new get_sonic_version_file() function isn't called anywhere. How do we benefit from mocking it in unit tests?

It will be used in 'sonic-utilities/config'.

sonic-net/sonic-utilities#1266

checkout config/main.py.

[smaheshm]

retest mellanox please

[smaheshm]

retest vsimage please

[smaheshm]

retest mellanox please

[judyjoseph]

Does all the current callers of this API, handle a return value of "None" ? If this is a mandatory file, we could raise an exception.

[smaheshm]

There is only one caller (sonic-net/sonic-utilities#1266). I'd say let's leave it to the caller to decide. Python will raise exception anyway.

[smaheshm]

retest mellanox please