Fix minimal_access_user_spec on staging

What does this MR do?

Fixes minimal_access_user_spec on staging by using admin api client for creating the user.

Related to failures:

• gitlab-org/quality/testcases#1134 (comment 494756719)
• gitlab-org/quality/testcases#1071 (comment 494760452)

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content