Etcd3 nb-db driver

Etcd3 has a brand new set of API, which supports pub/sub mechanism, and its performance is also promising. It is the time to upgrade etcd2 driver and implement its own pub/sub driver as well.

https://python-etcd3.readthedocs.io/en/latest/readme.html

A discussion of using etcd3 as the default tooz driver:
http://lists.openstack.org/pipermail/openstack-dev/2017-March/113885.html