Ignore EtcdResult from pylint.

It uses dynamic attributes (via setattr()) that can't be detected
by static analysis.

test/Dockerfile

@@ -7,4 +7,4 @@ RUN \
     pylint
 
 WORKDIR /
-ENTRYPOINT ["pylint", "/etcdlib.py"] 
+ENTRYPOINT ["pylint", "--ignored-classes=etcd.EtcdResult", "/etcdlib.py"]