While reviewing the hooks document, I realised we are not correctly
exporting the instance properties.

This patch fixes:
  - export the disk and disk template in all LUs, not only (hardcoded)
    in the instance create
  - removes the instance create INSTANCE_ prefix on some non-instance
    variables (those are LU-related, not instance-related)
  - adds a couple of more variables to other LUs

The hook document will be updated in a separate patch.

Reviewed-by: ultrotter