Avoid stubbing Gitlab.config.gitlab.host in the tests

We have this in some of the tests:

allow(Gitlab.config.gitlab).to receive(:host).and_return('www.example.com')

This feels like something wasn't properly setup. I tried to put this but it doesn't work: (test still failed)

Capybara.default_host = "http://#{Gitlab.config.gitlab.host}"

The following discussion from !17676 (merged) should be addressed:

• @artofhuman commented on a discussion: (+2 comments)

  It fails because here https://gitlab.com/gitlab-org/gitlab-ce/blob/master/app/controllers/sessions_controller.rb#L105 redirect_allowed_to?(redirect_uri) returns false and referer url not stored. In test env we have config value

  [2] pry(main)> Gitlab.config.gitlab.host
  => "localhost"

  but in tests host is example.com