Add ability to override scenario suite via environment variable

What does this MR do and why?

Add ability to override E2E test suite class name and not use the one hardcoded in gitlab-qa scenario class. This is needed when in particular cases pipeline setup requires running a different set of tests (using tags or spec files directly is problematic because scenario classes in e2e test framework serve as SSOT for both calculating runtime for dynamic job scaling and then running actual test suite).

Related to: Add CreateStage scenario class to limit test su... (gitlab-org/quality/analytics/team#22 - closed)

MR acceptance checklist

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