Update constants.py

[samchecc]

What does this PR do?

This PR will avoid overriding IAM permissions access level classified by aws based on the user choice. If the end user provided an enviroment variable the default override file won't be used.

The reason of this change is that i would like to query aws iam permissions as it's from aws documentation without overriding. So i have added this option, where i can specify an empty file to avoid the default override file. For further details please refer to this opened isse --> https://github.com/salesforce/policy_sentry/issues/410

What gif best describes this PR or how it makes you feel?

Avoid overriding AWS IAM permissions dynamically during the initialization

Completion checklist

• [ x] Additions and changes have unit tests
• [ x] Unit tests, Pylint, security testing, and Integration tests are passing.. GitHub actions does this automatically
• [ x] The pull request has been appropriately labeled using the provided PR labels

#410

[salesforce-cla[bot]]

Thanks for the contribution! Before we can merge this, we need @samchecc to sign the Salesforce.com Contributor License Agreement.