ObjectStorage Prefix specified in BackupStorageLocation is ignored

[jacksgt]

Hello,

Velero's BackupStorageLocation (.spec.objectStorage.prefix) as well as OADP's DataProtectionApplication (.spec.backupLocations[].velero.objectStorage.prefix) allow setting a (optional) prefix for files uploaded into the S3 bucket:

Velero assumes it has control over the location you provide so you should use a dedicated bucket or prefix. If you provide a prefix, then the rest of the bucket is safe to use for multiple purposes.

apiVersion: velero.io/v1
kind: BackupStorageLocation
metadata:
  name: default
spec:
  provider: aws
  objectStorage:
    bucket: myBucket
    prefix: myPrefix

https://velero.io/docs/v1.13/api-types/backupstoragelocation/

Our BackupStorageLocation is configured with such a prefix, but the plugin currently ignores this. This leads to conflicts in the S3 bucket since ImageStream data is always uploaded to docker/registry/v2/... (when using the PluginRegistry). This appears to be the problematic part of the code: https://github.com/openshift/openshift-velero-plugin/blob/95ab2b038f0170ab6b0fa70bcb2d28268b312739/velero-plugins/imagestream/registry.go#L133

An additional environment variable REGISTRY_STORAGE_FILESYSTEM_ROOTDIRECTORY=${BSL.spec.objectStorage.prefix} should be added to address this (see example in the (registry configuration documentation](https://distribution.github.io/distribution/about/configuration/))