Add support for https endpoints

https://github.com/kragniz/python-etcd3/blob/master/etcd3/client.py#L108

As of right now if no certs are provided it's assumed it's an insecure connection. We should be setting the default grpc channel credentials in the event https is present without certs.

Example:
https://github.com/davissp14/etcdv3-ruby/blob/master/lib/etcdv3/connection.rb#L41-L47

Start in etcd3/client.py around line 108 and compare the connection behavior with the linked etcdv3-ruby example. Check how an https endpoint is handled when no certificates are supplied, then verify that the default gRPC channel credentials are used in that case.

Written by the indexing model from the issue text.