fix(rackspace): add RSA note for keypairs
fix(rackspace): add RSA note for keypairs

michelleN/docs,wli6/docs,iankronquist/docs,jonboulle/docs,hkjn/docs-1,ksmaheshkumar/docs-2,endocode/coreos-docs,coreos/docs,mjg59/docs,robszumski/docs,atxwebs/docs,mischief/coreos-docs,skynet/docs,bq-xiao/docs,jdloft/docs,joshix/docs,ionave/docs,marineam/coreos-docs