Remove restrictions on initrd generators.

This restriction is not really enforced anywhere else, and callers
should have control over which tool they use as long as it is supported
by their system and kernel-install.

roles/uki_config/meta/argument_specs.yaml

@@ -7,8 +7,7 @@ argument_specs:
     options:
       uki_config_initrd_generator:
         type: str
-        choices:
-          - dracut
+        description: The tool kernel-install calls to generate an initramfs.
         default: dracut
 
       uki_config_mok: