Geo: internally synchronize Git LFS blobs in object storage

Git LFS support was added in %9.0 by gitlab-org/gitlab-ee!1382 (see also gitlab-org/gitlab-ee!1530). But since the implementation of gitlab-org/gitlab-ee!2760 in %10.0, LFS objects also can be stored on object storage (S3) (see also gitlab-org/gitlab-ee#2841).

Ideally LFS objects stored in object storage should be synchronized using one of the various object synchronization tools, but we may need to consider automatically handling this in the future.

Rationale

Now LFS support on S3 object storage is implemented, we should select an S3 compatible daemon that supports replication / multi copy (like Riak or Ceph), integrate it into our omnibus install, create documentation on how to setup replication and make some minimal changes to re-enable LFS read-only support on secondary Geo.

As temporary fix, LFS object on object storage are ignored, but a permanent solution to actually sync them should be provided.