jx gitops image to support registry/arch modification

we should add CLI arguments to allow the registry prefix on image strings to be modified for Deployments / Jobs.

Eg

```bash
jx gitops image —from-registry gcr.io —to-registry my-registry-host
```

would then replace any files in `content-root/**.yaml` which have a container image like `gcr.io/myorg/myimage` => `my-registry-host/myorg/myimage`

also it would be nice to be able to add a `—arch arm64` argument to modify the image to reference the arm64 architecture