[OpenStack-Heat] Add link to the OpenStack CLI install documentation

cluster/openstack-heat/util.sh

@@ -39,6 +39,8 @@ function verify-prereqs() {
   else
     echo "${client} client does not exist"
     echo "Please install ${client} client, and retry."
+    echo "Documentation for installing ${client} can be found at"
+    echo "http://docs.openstack.org/user-guide/common/cli-install-openstack-command-line-clients.html"
     exit 1
   fi
  done