Update rolling_update.md
Update rolling_update.md

modify the document, add the checks before rolling.
aledbf/kops,robinpercy/kops,kubernetes/kops,robinpercy/kops,justinsb/kops,kubernetes/kops,gambol99/kops,aledbf/kops,gambol99/kops,justinsb/kops,aledbf/kops,gambol99/kops,aledbf/kops,robinpercy/kops,robinpercy/kops,kubernetes/kops,gambol99/kops,justinsb/kops