E2E: Add ability to set gitlab-url via environment variable

What does this MR do and why?

Describe in detail what your merge request does and why.

Add ability to set up gitlab url through environment variable

• default to gdk when no environnment set
• do not redefine gitlab_address twice in scenario attributes when it is already defined via named option
• make setting url's more consistent, only accept gitlab url as first argument to reduce confusion
• refactor some of the unit tests to properly mock external dependencies

Next iteration: move all logic that doesn't require cli option parsing from Template to spec_helper.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

First part of many incoming changes for: https://gitlab.com/gitlab-org/quality/team-tasks/-/issues/1034