code: def push(self, cart, env=None, callback=None):
          
          juicer.utils.Log.log_debug("Initializing push of cart '%s'" % cart.cart_name)
  
          if not env:
              env = self._defaults['start_in']
  
          cart.current_env = env
          self.sign_cart_for_env_maybe(cart, env)
          self.upload(env, cart, callback)
          return True
docs: `cart` - Release cart to push items from
          `callback` - Optional callback to call if juicer.utils.upload_rpm succeeds
  
          Pushes the items in a release cart to the pre-release environment.