migrations for database nop should only be done on nop database

nop/dbrouter.py

+class NopRouter(object):
+    def allow_migrate(self, db, app_label, model_name=None, **hints):
+        if app_label == 'nop':
+            return db == 'nop'
+        return None

nop/migrations/0001_initial.py

@@ -6,6 +6,8 @@ from django.db import migrations, models
 
 class Migration(migrations.Migration):
 
+    app_label = 'nop'
+
     dependencies = [
     ]
 

pv/settings.py

@@ -21,6 +21,8 @@ DATABASES = {
     }
 }
 
+DATABASE_ROUTERS = ['nop.dbrouter.NopRouter']
+
 TIME_ZONE = 'Europe/Vienna'
 
 LANGUAGE_CODE = 'de'