Remove getcwd to load WAAgent properly and not fall to backup

[gopaljain00]

This PR is a fix for this issue: https://msazure.visualstudio.com/One/_workitems/edit/36801896
(This WI has response from GuestAgent team which provides insights on the possible errors.)

VMBackup uses a library called WAAgent (https://github.com/Azure/WALinuxAgent)
This library comes bundled with the VMBackup extension (present in VMBackup\main\WaagentLib.py), and is loaded by a utility file (VMBackup/main/Utils/WAAgentUtil.py)

WAAgentUtil.py loads WAAgent in 2 steps.
Step-1: Try to load the bundled file by constructing its path -
agentPath = os.path.join(os.getcwd(), "main/WaagentLib.py")

Step-2: If above path is not resolved correctly, check if this library is present in the user's VM in the directory:
agentPath = '/usr/sbin/waagent'

Ideally, Step-1 should work, and we should not need to reach fallback (i.e Step-2).

The issue here is the path resolution in Step-1 is fragile. It will only work when executed from extension root (as it uses getcwd()). Which might result in loading the fallback.
And it is not guaranteed that the fallback works as expected with our extension as intended as it is coming from User's VM so there can be version mismatch, etc (resulting in missing dependencies such as ShellUtil not defined).

In this PR, I've added changes to:

1. Solidify the path resolution from the Util file to the bundled WAAgent library - This should fix the shellUtil errors

Some additional changes done:

1. Similar issue was present for loading handle.sh from handle.py - This should improve the errors related to the extension not being initialized successfully.
2. We are loading the environment variable PYTHONPATH. If the User's VM does not have this variable set, it will throw an error, therefore added a fallback value of empty string.

[gopaljain00]

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[OMJayswal]

This line broke in the last release which was reverted as a part of this rollback.
I suggest you to either checkout from main and then merge your changes(might need conflict resolution) or removed what's undone in this PR
Also, remove files with whitespace changes only from the PR.

[gopaljain00]

Got it. I will see if we can reproduce the same (+ fix) in UTs.