[osbs] Add a phase to build images in OSBS
    
    It will take RPM repo from a variant in this compose and a Dockerfile
    from configured git and use it to build an image.
    
    The build images are uploaded to some a Docker registry by OSBS and are
    not directly part of compose (because there is no export function).
    There is a new metadata file `osbs.json` that has some information that
    can be used to find the image.
    
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>