Missing resticRepoPrefix config key

[martinkubrak]

What steps did you take and what happened:
Trying to use Restic for NFS-backed volume is not possible because Alicloud Velero plugin does not accept resticRepoPrefix config key. The ResticRepository custom resource fails with restic repository prefix (resticRepoPrefix) not specified in backup storage location's config

What did you expect to happen:
Alicloud Velero plugin should work with Restic.

Anything else you would like to add:
See this issue

Environment:

• Velero version: (use ark version): v1.6.2
• Kubernetes version: (use kubectl version): v1.20.4-aliyun.1