Make 'Nightly' the milestone for Atomic, Cloud, Docker composes
    
    Per all the changes to prefer identifying these via 'dist', it
    doesn't make sense for instances of these classes to have the
    compose type as their 'milestone' attribute, I don't think. So
    let's make it 'Nightly'. This could just *possibly* be a hostage
    to fortune if we come up with *other* nightly composes with the
    same dist name in future, but I can't think of a 'safer' name
    that isn't hideously ugly (Postreleasenightly? Ugh.)
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>