json: use ujson to dumps
json: use ujson to dumps

This should improve encoding speed.

Change-Id: I0492d42a80ffae6f128b9ca5875c651a6e03e59e

leandroreox/gnocchi,leandroreox/gnocchi,gnocchixyz/gnocchi,gnocchixyz/gnocchi,sileht/gnocchi,sileht/gnocchi