Issues with TLS client authentication #20

While setting up Calico in our OpenStack deployment we've encountered issues when trying to use TLS authentication to access etcd cluster.

To reproduce:

1. Setup etcd cluster with TLS client authentication
2. Create new client c = Etcd3Client(host="server", protocol="https", ca_cert="server_ca.crt", cert_cert="user.crt", cert_key="user.key")
3. try to get some random key c.get("random_key")
4. You should get TLS alert bad certificate or CERTIFICATE_VERIFY_FAILED if you don't have server_ca in your system trust.

When looking through the source code we've noticed that the TLS parameters are stored in self.kwargs and then never used.

When edited to actually set the values on the session object, the TLS auth started working.

Issue appeared on both Ubuntu Xenial (Python 2.7.12, OpenSSL 1.0.2g) and Fedora 27 (Python 2.7.14, OpenSSL 1.1.0h).