QA: Sign in as an admin account when adding a license

What does this MR do?

When performing LDAP tests the LDAP user can't add a license because it doesn't have admin rights. Rather than create an Admin LDAP user and have to use it for all LDAP tests, this MR uses an admin user to add the license.

By default GDK's root user will be used but it's also possible to specify an admin user via GITLAB_ADMIN_USERNAME and GITLAB_ADMIN_PASSWORD environment variables.

The change to the factory method that adds a license is in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/7310

What are the relevant issue numbers?

gitlab-qa#321 (closed) and https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21354

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides