coreos: drop requirement on stream key
    
    RoboSignatory actually doesn't need to know the stream being signed at
    all. I think the original rationale for this was so that we could tell
    which stream was being signed for informational purposes. Nowadays: (1)
    RoboSignatory returns all the keys in the request message in its
    response, (2) FCOS stream information is encoded in the version ID
    already, and (3) we're going to add idempotency request ID tokens to
    track individual requests.
    
    Signed-off-by: Jonathan Lebon <jonathan@jlebon.com>