Adds packages objectstorage to backups.
requested to merge matthias.vandemeent/CNG:charts/gitlab-900-add-backup-for-maven-packages into master
Basically a copy-paste of how lfs and uploads are handled.
Fixes charts/gitlab#900
cc gitlab-com/marketing/community-relations/contributor-program/gitlab-hackathon/q4-2018-hackathon#1