Fixed Rackspace example

As per https://github.com/mitchellh/packer/issues/1142, it's critical to add openstack_provider: rackspace in order to get anything to work reliably with Rackspace and Packer in 6.0.

Question: should the openstack_provider field now be marked as required?
Merge pull request #1347 from kanerogers/patch-1

website: Fixed Rackspace example