Enable using login modifier with only partial path

[malaskowski]

Description

Currently, the login modifier requires passing the full login-page URL in the properties.

Motivation and Context

Passing full URL to the login page is sometimes difficult, especially when a single suite is reused among multiple domains.
This PR enables running login modifier with only part of the login page, and the full URL is built using domain (the same logic as url works).

Upgrade notes (if appropriate)

This feature is backward compatible. Proper updates were made in the wiki docs.
The logic behind the change is:

• if the login-page starts with http prefix, then the old logic is triggered (full login page URL)
• otherwise, login-page is treated as a path, and domain is added in the front (see unit tests for details).

Types of changes

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

Checklist:

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

---

I hereby agree to the terms of the AET Contributor License Agreement.

[annabudziszewska]

Tested and accepted
version: 3.3.1.SNAPSHOT