Add support for authentication

[Heitor]

etcd support authentication using username and password.

etcd3-py[0] supports that, and it is possible to subclass etcd3gw to add user:pass auth as well[1].
Subclassing etcd3gw is not a solution for new users.

Do you think it is possible to add user:pass auth to etc3gw?

Thanks,

[0] https://github.com/Revolution1/etcd3-py
[1] https://github.com/projectcalico/calico/blob/master/networking-calico/networking_calico/etcdv3.py#L388-L472