This commit replaces the update_inventory.py script with a Nix module
based on Flake Parts [1] which defines all configuration options
including types and default values.

Only values that are exposed by the module can be set in the
configuration. Exposed values have been removed from the default/
main.yaml in order to have only a single source of truth. As a result,
all variables (set or not) are rendered into the inventory.

The cluster initialization is now backed by Nix Flake templates.
Template files for new clusters can be found at ./nix/templates/
cluster-repo.

The cluster repository layout is changed in such a way that the
./config directory is completely handled by the user and the ./inventory
directory is completely generated and may be ignored from the VCS. A
./state directory is added which is both input and output of the
inventory generation and which has to be added to VCS.

                   +---------+
                   | ./state |
                   +--+---^--+
                      |   |
               +------v---+---------+
+----------+   |                    |   +-------------+
| ./config +--->     Nix module     +---> ./inventory |
+----------+   |                    |   +-------------+
               +--------------------+

Resource request and limit handling has been changed such that
unflattened arrays are used instead of individual options for each
value.

[1] https://flake.parts