postmoogle: set safe defaults to POSTMOOGLE_USERS

roles/matrix-bot-postmoogle/defaults/main.yml

@@ -85,7 +85,8 @@ matrix_bot_postmoogle_maxsize: '1024'
 # - @another:example.com
 # - @bot.*:example.com
 # - @*:another.com
-matrix_bot_postmoogle_users: []
+matrix_bot_postmoogle_users:
+  - "@*:{{ matrix_domain }}"
 
 # A list of admins
 # Example set of rules: