Add optional Velero backup to chart

[gabor-boros]

At the moment, we are not providing any backup solutions for the cluster. Although a databases can be backed up separately, and most of the resources can be recreated with ease, having a safety net in terms of a backup would be essential on a production grade.

At OpenCraft, we are using Velero to do K8s backups and exporting them to an S3-like storage. This works really well, and we had the opportunity to test it in a real situation as well upon a cluster destroy.