Add Rackspace Cloud DNS Resource

This will be a temporary solution to have a resource for DNS until
Designate is incubated. Also currently work is in progress for a
new API for Designate.
Until then this will serve as a resource for Rackspace Cloud DNS.

Implements: blueprint rax-dns-resource
Change-Id: Ief0c478b9e40f6f9446b386243400f062a941a36