builder: Generate ostree summary metadata file when updating repository

[jwennerberg]

When the ostree repository is created and/or updated in the builder role, a ostree summary metafile should be generated.

The summary file is optional but certain integrations like the Cockpit OStree page look for this file. See example below.

Something like this should be enough:
/usr/bin/ostree summary --repo=/var/www/html/{{ builder_blueprint_name }}/repo --update

Warning in Cockpit when the remote repo is missing the summary file: