improve app-task s.t. a uwsgi-app can run without a database

uwsgi-python/tasks/app.yml

@@ -42,14 +42,15 @@
     - "{{ app.app }}"
     - "{{ app.instance }}"
 
+- block:
   - include: sqlite.yml
-  when: app.db == "sqlite"
-
+    when: app_db_type == "sqlite"
   - include: mysql.yml
-  when: app.db == "mysql"
+    when: app_db_type == "mysql"
 
   - include: postgres.yml
-  when: app.db == "postgres"
+    when: app_db_type == "postgres"
+  when: app_db_type is defined
 
 - name: ensure we have a group
   group: