feat(openstack): Add note on using config drive.
feat(openstack): Add note on using config drive.

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