Add 'short' key to post-release compose fedmsgs
    
    The various post-release composes - two-week Atomic, Cloud, and
    Docker - all send out identical fedmsgs. The other commits in
    this PR at least ensure the fedmsgs sent after the compose is
    done will have a `compose_id` key, allowing disambiguation that
    way, but fedmsgs sent *before* the compose is done cannot be
    told apart.
    
    This should solve that, without inventing new message topics:
    we add a `short` key to the message content which specifies the
    'shortname' of the compose (e.g. Fedora-Atomic), which is how
    we distinguish these composes from each other.
    
    We also remove a bunch of unused variables from these files,
    and revise the release-related variables as explained for the
    master branch in #485: `$DIST`, `$BRANCH` and `$BRANCHED` are
    all replaced by `$RELEASE` and `$RELEASE_TITLE`.
    
    This does duplicate the value from the pungi config file, but
    I'm not sure it's worth figuring out a bash-y way to parse the
    value out of the config file just to remove this duplication.
    And we already effectively did this when discovering
    `OLDCOMPOSE_ID` and `NEWCOMPOSE_ID`, as the path used there is
    dependent upon the shortname; to avoid extra duplication, I've
    changed those to use `$SHORT-$RELEASE_TITLE`.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>