Run Packer asynchronously

[milliams]

Currently more than half the Ansible time is spent waiting for Packer to finish. If we could start it running earlier in the process and run the rest of the tasks in the meantime it would reduce the time-to-ready to just that of running Packer.

This can probably be done with async tasks: https://docs.ansible.com/ansible/latest/user_guide/playbooks_async.html

Also see https://blog.crisp.se/2018/01/27/maxwenzin/how-to-run-ansible-tasks-in-parallel and https://toptechtips.github.io/2019-07-09-ansible_run_playbooks_tasks_in_parallel/