Support OS_INTERFACE and set V2 API as default
Support OS_INTERFACE and set V2 API as default

This makes the client work with OS_INTERFACE / --os-interface and
makes it use the version 2 as default for dns.

Change-Id: I8d8bdd39f852dbe87846673d9bffc95b17e9a3cd

openstack/python-designateclient