harmonize this with the documentation. Once every minute is way to often.

Every 15min should be fine just as the documentation suggests: http://doc.owncloud.org/server/5.0/admin_manual/configuration/background_jobs.html

Fixes this: https://github.com/owncloud/core/issues/4383

@DeepDiver1975 @schiesbn @PVince81 @blizzz