Executing remote commands at bootstrap using Terraform

It's a very common practice to have a set of initial commands executed right after bootstrap, even before the proper configuration management system such as Chef or Ansible takes responsibility. It can include immediate full updating of the OS, initial registration on discovery systems such as Consul, or initial addition of local DNS servers. It really shouldn't go farther than delivering a system in a slightly more advanced and expected state for the next configuration system to take over. Under no circumstance should it replace a proper configuration management tool.

In this recipe, we'll launch a CentOS 7.2 system, then fully update it so it's as secure as possible, install EPEL so we have a greater library of available packages, add the Puppet Labs Yum repository and install a Puppet agent, and add a different name server so our system is ready for the next step (which we won't cover here, as it's probably...