osbuild: add support for building ostree artifacts
    
    In order to start building Fedora IoT images with osbuild, we need to be able
    to pass ostree options from pungi to the koji's osbuildImage task.
    
    This commit adds support for it via new configuration options: ostree_url,
    ostree_url and ostree_parent.
    
    A test was added to cover these new options and they are were also added
    into the documentation.
    
    JIRA: COMPOSER-1702
    Merges: https://pagure.io/pungi/pull-request/1624
    Signed-off-by: Ondřej Budai <ondrej@budai.cz>