Adding capabitity to run tests on images from custom registry

What does this MR do and why?

I am currently working on a private GitLab fork project which also uses a private fork of omnibus-gitlab-mirror repository to build images. The orchestrated tests in the MR pipelines are all failing, as gitlab-qa is unable to access the container registry of the private Gitlab fork project as well as the private omnibus-gitlab-mirror project.

This MR adds capability for the user to pass in custom registry url as well as credrentials for gitlab-qa to pull images from private project registries.

The new environment variables introduced are GITLAB_CUSTOM_ACCESS_TOKEN , GITLAB_CUSTOM_USERNAME and QA_CUSTOM_REGISTRY.

Tested by passing custom parameters : https://gitlab.com/gitlab-org/gitlab-qa/-/pipelines/1551531378

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