Add matrix_dendrite_guests_disabled

roles/custom/matrix-dendrite/templates/dendrite.yaml.j2

@@ -189,7 +189,7 @@ client_api:
 
   # Prevents new guest accounts from being created. Guest registration is also
   # disabled implicitly by setting 'registration_disabled' above.
-  guests_disabled: true
+  guests_disabled: {{ matrix_dendrite_guests_disabled | to_json }}
 
   # If set, allows registration by anyone who knows the shared secret, regardless of
   # whether registration is otherwise disabled.