Remove some hardcoded matrix-postgres references

roles/custom/matrix-bot-maubot/tasks/validate_config.yml

@@ -1,10 +1,11 @@
 ---
 
-- name: Fail if required settings not defined
+- name: Fail if required maubot settings not defined
   ansible.builtin.fail:
     msg: >-
-      You need to define a required configuration setting (`{{ item }}`).
-  when: "vars[item] == ''"
+      You need to define a required configuration setting (`{{ item.name }}`).
+  when: "item.when | bool and vars[item.name] == ''"
   with_items:
-    - matrix_bot_maubot_unshared_secret
-    - matrix_bot_maubot_admins
+    - {'name': 'matrix_bot_maubot_unshared_secret', when: true}
+    - {'name': 'matrix_bot_maubot_admins', when: true}
+    - {'name': 'matrix_bot_maubot_database_hostname', when: "{{ matrix_bot_maubot_database_engine == 'postgres' }}"}