Native Nova Server compatibility for Cloud Server

Make the Rackspace Cloud Server resource compatible with the native
Nova Server resource so that a template author can replace
OS::Nova::Server with Rackspace::Cloud::Server, without making any other
changes, to make the resource use Rackspace Cloud Servers instead of
Nova.

Blueprint native-nova-cloud-server
Change-Id: I312ddc25e02843b0116b8a2cb88defef05ca88ac