add possibility to customize the velero-restic-credentials

**Describe the problem/challenge you have**
velero-restic-credentials is always set to `static-passw0rd`

**Describe the solution you'd like**
have a secret that is created if another value is specified. ex: `vault:velero...`

this would solve https://github.com/vmware-tanzu/velero/issues/1053 for people using helm
