Store composes in directories in `odcs-$COMPOSE_ID` format.
- The code is simpler now, because there is no need to make
differences between toplevel_work_dir and toplevel_dir. We
only use toplevel_dir now which points to `odcs-$COMPOSE_ID`.
- The `toplevel_url` is included in the API response, so it is
possible to use it to get to compose root directly from API.
- The full-path to compose can now be easily generated by external
scripts by simply knowing the compose id.
Signed-off-by: Jan Kaluza <jkaluza@redhat.com>