Support OpenStack object storage for direct_upload mode

As mentioned in https://gitlab.com/charts/gitlab/issues/1350, we only support AWS and Google for direct_upload.

This issue tracks the work needed to support OpenStack. It seems that OpenStack has a slightly different API in https://github.com/fog/fog-openstack/blob/03e889daf2cc457853e90aa69ce91962b9caf896/lib/fog/openstack/storage.rb.

1. Add OpenStack to SUPPORTED_DIRECT_UPLOAD_PROVIDERS in https://gitlab.com/gitlab-org/gitlab-ce/blob/master/config/initializers/direct_upload_support.rb.
2. get_object_url => get_object_http_url or get_object_https_url
3. delete_object_url => ?
4. put_object_url => ?
5. signed_url => ?