Use ostree-push to publish prepared repository

Switch to `ostree-push` instead of `rsync` allows to keep
all branches in single repository.

Naming convention for branches includes release number for now:
- apertis/${release}/${architecture}-${board}/${type}

Signed-off-by: Denis Pynkin <denis.pynkin@collabora.com>
Reviewed-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Differential Revision: https://phabricator.apertis.org/D7542