Changes to GitLab QA caused by force initial root password configuration

gitlab-org/gitlab#211328 (closed) issue will force root password configuration at install time. For now, the decision is to randomly set a password if none is provided. Then a gitlab-ctl command could be used to retrieve it.

This is a breaking change and we need to explore what should be done to make the switch smooth. Not an exhaustive list:

• Update default_password in qa/qa/runtime/user.rb.
• Require initial password in GitLab QA pipelines that spin up GitLab instances - nightly, master, package-and-qa, etc - using gitlab_rails['initial_root_password'] Omnibus config, similar to GITLAB_INITIAL_ROOT_PASSWORD.
• Update GitLab QA documentation that mentions the initial root password.