Inventory: add missing settings to build the test machine

machines/minimal_inventory_machine/README.md

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+This settings exists only for testing purpose to make the machine bootable
+
+It serves as a gap close, until we have the required boot and filesystem modules ready for the inventory.

machines/minimal_inventory_machine/settings.json

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+{
+  "fileSystems": { "/": { "device": "/dev/null" } },
+  "boot": { "loader": { "grub": { "device": "/dev/null" } } }
+}