title: call async function in main function
body: <p>I would like to know if it's possible to call an async function <code>def get_all_allowed_systems</code> in <code>create_app</code> function so that I have access to the database entries of <code>ALLOWED_SYSTEMS</code> populated by get_all_allowed_systems call. I have a limitation that I can't make <code>create_app</code> as async function.</p>
  <pre><code>async def get_all_allowed_systems(app):
      global ALLOWED_SYSTEMS
      operation = prepare_exec(app.config.get_all_systems_procedure)
      ALLOWED_SYSTEMS = (await app['database'].execute(operation)).all()
  
  def create_app():
      app = App(config=Config)
      app['database'] = AioDatabase(**app.config.dict('db_'))
      app['app_database'] = AioDatabase(app.config.app_db_url)
      get_all_allowed_systems(app)
      print(ALLOWED_SYSTEMS)
  </code></pre>