fix(cloud-providers/rackspace): fix example
fix(cloud-providers/rackspace): fix example

The example is broken, as `OS_AUTH_SYSTEM = rackspace` is required when communicating with the Rackspace endpoint. Additionally, updated language referencing the `OS_TENANT_NAME` and how to locate it for Rackspace.
michelleN/docs,wli6/docs,ksmaheshkumar/docs-2,atxwebs/docs,marineam/coreos-docs,endocode/coreos-docs,bq-xiao/docs,hkjn/docs-1,mischief/coreos-docs,ionave/docs,robszumski/docs,mjg59/docs,jdloft/docs,coreos/docs,iankronquist/docs,skynet/docs,joshix/docs,jonboulle/docs