Geo: Add QA test for Alternate URL functionality

Follow up https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/10358#note_154474712:

If we don't have an e2e / QA test for this it might be worth creating an Issue for the future.

• Test "Removing Alternate URL from a Geo secondary disables OAuth at that URL" to ensure we never regress https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/10358.
• Also may as well test "Adding an Alternate URL to a Geo secondary enables OAuth at that URL".

cc @ashmckenzie