Ensure designate-api has policy initialized

We create a new WSGIService class, which inherits from both
the base designate Service class, and the openstack-common
wsgi.Service class.

This allows our common service initialization code to be
executed.

Closes-Bug: 1366766
Change-Id: I60ed6222a1e9bd54329545767ee329b54876e73a
Merge "Ensure designate-api has policy initialized"