restic-helper-image needs to be customizable for local container registries

[AhmedBenAbid]

Describe the problem/challenge you have
Hello,
We are using PKS as a Kubernetes installer on vSphere in a air-gapped environnement, so all Docker images need to be imported to a local registry.
The vSphere version we're currently using doesn't support Volume Snaphot, so we choose Restic integration for PV backups.

We are using the CLI to install the velero, which allow us to choose velero's and restic's images, but not the restic-restore-helper image (which can be configured later using a ConfigMap)
Also, we cannot choose hostPath path on installation

Describe the solution you'd like
It would be practical to add these two parameters as cli flags:
--restic-helper-image
--restic-hostpath

Environment:

• Velero version: Version: v1.5.1
• Kubernetes version: v1.17.0
• Kubernetes installer & version: PKS 1.7
• Cloud provider or hardware configuration: vSphere
• OS (e.g. from /etc/os-release):

Thank you for your attention.

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[nrb]

I think we can reliably detect TKGI's preferred restic hostpath with information provided in #2767. The helper image would definitely be useful to add, though.

[nrb]

@carlisia Can we figure out how to do this with the Carvel work? I'd rather not add more options to velero install, but I think these options should be exposed.