add velero to cern-magnum

Added velero to cern-mangum, configurable via the following labels:

• backups_enabled (bool)
• backups_s3_bucket (str) - name of s3 bucket to store backup data to. Users need to create this ahead of time.
• `backups_s3_credentials (str) - access credentials for s3 bucket. These will be stored as a secret in the cluster. Sample of credentials file provided below.

[default]
aws_access_key_id = "s3#access_key>"
aws_secret_access_key = "s3#secret_key>"

Will draft updates to public documentation today and submit MR once this is merged in.

Closes #85 (closed)