Cloud-init is the package that handles early initialization of a cloud instance. Cloud-init allows you to customize an instance at boot using a user-data script. This allows you to override vendor-data scripts like the forced update on initial boot, add software, add git repositories, add users, and many other actions.
To use a cloud-init script on the command line, you add
to the openstack server create command.
When doing a cloud-init script, it's crucial to start it with #cloud-config so the interpreter knows that it's not just a standard shell script.
There are a number of examples of cloud-init scripts here: http://cloudinit.readthedocs.io/en/latest/topics/examples.html
final_message: "Boot completed in $UPTIME seconds"