Close the NBD socket when testing it so it won't mess up qemu.

/reviewed-on nexedi/slapos!516

- Add `buildout.hash.cfg` file so this way we can use the useful `update-hash` script
- Refactor a bit the `software.cfg` file sections as it was repeating itself

/reviewed-on nexedi/slapos!512

Extend the backend with Timeout configuration via headers and use it
to prove, that request taking more than try_duration is correctly served.

Also prove that try_duration and try_interval are correct passed to the slave
configuration.