Login modifier reads from env

[malaskowski]

Description

Login modifier can read login and password from environment variables.

Motivation and Context

Fixes #512

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• [x] My code follows the code style of this project.
• [x] I have reviewed (and updated if needed) the documentation regarding this change

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