Add a new assembler that takes a file system tree that is already
conforming to the ostree system layout[1], creates a new repository
in archive mode and commits the file system tree to it. Afterwards,
a reference is created with the value supplied in `ref`.
The repository is located at the `/repo` directory and additional
metadata is /compose.json which contain the compose information.
Currently uses rpm-ostree to do the actual committing. In the future
this might change to plain ostree.
[1] https://ostree.readthedocs.io/en/stable/manual/adapting-existing/
