nightly: include shortname in fedmsgs, clean up version vars
    
    The main point here is to include the compose 'shortname' in
    compose fedmsgs, as #484 does for post-release compose types.
    While doing this I noticed there's rather a lot of mess and
    cruft in the various version-related variables here (and a few
    others).
    
    `GIT_BRANCH` variable is never used anywhere any more, so it's
    removed.
    
    `BRANCH` was never used anywhere, so it is gone. (It appears
    to have been added in a25b08c7 but never used). There seemed
    to be no useful distinction between `DIST` and `BRANCHED`, and
    both actually seem to be used mainly to specify a release
    version. The exception is that `BRANCHED` was set to
    "Modular-Bikeshed" in nightly-modular.sh , but there is no
    justification for this that I can find, and its only practical
    use was as the value of the `branch` key in fedmsgs. So far as
    I can figure out, the original purpose of the `branch` key was
    for when we sent out `compose.branched.(foo)` fedmsgs - it
    was e.g. 'f18' when Branched was Fedora 18. As we now don't
    send out `compose.branched` messages any more, but include the
    release number in the message topic for Branched messages
    (e.g. `compose.27`), this purpose no longer applies. So the
    key just doesn't seem to have any particular use at all any
    more, and I don't think changing its value will have any real
    consequences.
    
    Thus, `BRANCH`, `BRANCHED` and `DIST` are replaced by `RELEASE`
    and `RELEASE_TITLE`, which specify the release version. Their
    values will be identical for numerical release versions, but
    for Rawhide and Bikeshed we need to know both the lower-case
    and title-case versions of the name; RELEASE_TITLE is the
    title-case version.
    
    We use `$SHORT-$RELEASE_TITLE` to replace several instances
    of hardcoding in the scripts (e.g. Fedora-Modular-Bikeshed).
    
    The `ATOMIC`, `ATOMICREPO` and `ATOMICDEST` variables that were
    blindly copied from nightly.sh to nightly-modular.sh, but never
    actually used in it, are also removed.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>