rancher: rework questions.yaml

Rework questions.yaml incoorporating user feedback

- Ensure `example.com` is used as placeholder url where applicable
- Reorder and re-organize questions into sensible groups
- Utilize Rancher's builtin types where possible to generate drop-down
  menus of existing K8s objects instead of requiring text input.
  This applies to the storage class and secrets.

fixes: aquarist-labs/s3gw#341
Signed-off-by: Moritz Röhrich <[email protected]>