inventory: Rename file to add file extension

This is a YAML file, make ansible-lint (and me) happy and add a proper
file extension. Also helps editors to pick correct syntax highlighting.

README.md

@@ -14,7 +14,7 @@ ansible-galaxy install -r requirements.yml
 
 ## Call with
 ```bash
-ansible-playbook -i inventory --ask-vault-pass main.yml
+ansible-playbook -i inventory.yml --ask-vault-pass main.yml
 ```
 
 You need to provide a user with sudo rights and the vault password.