Remove existing "embedded instance" options:
- `embedded-sr`
- `embedded-sr-type`
- `embedded-instance-parameters`

Instead introduce `one-time-embedded-sr`:
- Supply the given SR, but only the first time.
- Create a supply/request script for the user.

This parameter is only taken into account the first time it's used.
It __doesn't__ automatically request an instance. It doesn't call
the request script created for the user.