Currently gnocchi metricd is really slow making some tests timeout,
because the tooz backend is mysql.
And the futur autoscaling will reach the max connections of mysql
by creating many tooz locks.
This change switches to redis, this is one of the recommended backend to
use (mysql is not).
Change-Id: Iae81848e883f74b7f8f8f4cad58c2168e01b7321
