refactor: move out debootstrap config to its own class

Note: this is an API-breaking change.

Some ganeti users might not be using ganeti-instance-debootstrap as the OS template for creating instances.

The module currently lets users avoid installing and configuring this OS template, but the whole code path that gets avoided is pretty big. The number of options in the main class pertaining to this OS template is also becoming quite large.

In order to declutter the both the config and the main class, let's push the configuration code out to its own separate class, and let's move all parameter definitions for the template to this class and bundle them up in the main class as one hash.

This is a reroll of !36 (closed) that i mistakenly closed and that we can mysteriously not reopen. The only task left from there is to update the upgrade docs. There was a bit about renaming a confusing parameter too, however.