Packagecloud uploads

This MR makes changes to omnibus-gitlab to allow for uploading packages to a Packagecloud server.

• keep uploading packages to S3 but also push to packagecloud;
• rename the packages from 'gitlab' to 'gitlab-ce'/'gitlab-ee'; the new packages will automatically replace 'gitlab' packages;
• make a DB-only backup on every package upgrade in the pre-install script
• automatically reconfigure in the post-install script (including DB migrations) unless /etc/gitlab/skip-auto-migrations exists
• around the automatic reconfigure during post-install, stop/start services to protect data integrity during migrations

cc @marin @sytses