[sonic-py-common] get_platform(): Refactor method of retrieving platform identifier

[jleveque]

- Why I did it
Applications running in the host OS can read the platform identifier from /host/machine.conf. When loading configuration, sonic-config-engine needs to read the platform identifier from machine.conf, as it it responsible for populating the value in Config DB.

When an application is running inside a Docker container, the machine.conf file is not accessible, as the /host directory is not mounted. So we need to retrieve the platform identifier from Config DB if get_platform() is called from inside a Docker
container. However, we can't simply check that we're running in a Docker container because the host OS of the SONiC virtual switch is running inside a Docker container.

- How I did it

• Refactor get_platform() to:

  1. Read from the PLATFORM environment variable if it exists (which is defined in a virtual switch Docker container)
  2. Read from machine.conf if possible (works in the host OS of a standard SONiC image, critical for sonic-config-engine when loading config)
  3. Read the value from Config DB (needed for Docker containers running in SONiC, as machine.conf is not accessible to them)

• Also fix typo in daemon_base.py

• Also changes to align get_hwsku() with get_platform()

- How to verify it
Do the following from:

1. The host OS of a standard SONiC image
2. Inside a Docker container running on a standard SONiC image
3. Inside the sonic-docker-vs virtual switch container

root@sonic:/# python2
Python 2.7.16 (default, Oct 10 2019, 22:02:15) 
[GCC 8.3.0] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> from sonic_py_common.device_info import get_platform
>>> get_platform()

Ensure the platform identifier is correct, and is not None.

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

• 201811
• 201911
• 202006