knife batch is a wonderful little plugin for executing commands a la knife ssh, but doing it in groups of n with a sleep between execution iterations.
gem install knife-batch
knife batch works exactly like knife ssh but with a couple of additional options.
knife batch "role:cluster" "whoami" -B 10 -W 5 will execute whoami against 10 servers with a sleep of 5 seconds in between.
-B INTEGER defines how many servers at max will be batched.
-W INTEGER defines the time to sleep in between executions.