As gitlab-workhorse is now serving everything, it proxies unknown-to-it
requests to unicorn, and that returns redirect to /users/sign_in for
/non-existent, not 403.

Adjust the promise and just verify whether we can get /static.css

NOTE nginx promise now effectively depends on gitlab-workhorse being up
    and running - because nginx is now configured to be only an SSL
    terminator and all requests processing is done by gitlab-workhorse and
    services below it.

    If in the future we'll need a pure nginx-is-up promise, we can
    introduce something to nginx config and verify that particular http
    endpoint.

/cc @kazuhiko, @jerome