I provision jenkins slaves by using chef. Jenkins slaves have to access to the github.com, heroku.com and so on via ssh. They have to know know_hosts.

gistfile1.sh

echo "cookbook 'ssh_known_hosts', github: 'opscode-cookbooks/ssh_known_hosts'" >> Berksfile
berks install
berks upload
knife data bag create ssh_known_hosts
knife data bag from file ssh_known_hosts data_bags/ssh_known_hosts/github.json
knife data bag from file ssh_known_hosts data_bags/ssh_known_hosts/heroku.json
knife role from file roles/jenkins-slave.rb

github.json

{
  "id": "github",
  "fqdn": "github.com"
}

heroku.json

{
  "id": "heroku",
  "fqdn": "heroku.com"
}

jenkins-slave.rb

# roles/jenkins-slave.rb
name "jenkins-slave"
description "jenkins slave"
run_list(
  "recipe[ssh_known_hosts::default]",
)