atomic-host-installer: Switch to storing the OSTree repo in /
    
    We hit an issue with having signed commits in the ISO, since
    Joliet filenames are severely limited and `$hash.commit` and `$hash.commitmeta`
    conflict.
    
    What we want to do long term is change Lorax to use OSTree itself for
    Anaconda - that would get us deduplication, and that'd be a big deal
    given that we're pushing 900MB now.
    
    This doesn't implement dedup, but it moves us in that direction by
    initializing a `bare` mode `/ostree/repo` in the Anaconda environment,
    which we pull from.
    
    NOTE NOTE NOTE
    --------------
    This requires `--rootfs-size=3` and switching both templates to `--add-template`.
    Like this:
    
    ```
    lorax --rootfs-size=3
          --add-template=$(pwd)/fedora-lorax-templates/atomic-installer/lorax-configure-repo.tmpl
          --add-template=$(pwd)/fedora-lorax-templates/atomic-installer/lorax-embed-repo.tmpl
          --add-template-var=ostree_osname=fedora-atomic
          --add-template-var=ostree_ref=fedora-atomic/25/x86_64/docker-host
          --add-template-var=ostree_repo=http://192.168.122.1/fedora-atomic/repo-upstream
    ```
    
    Signed-off-by: Colin Walters <walters@verbum.org>