The Ansible Vagrant provisioner has a race where the inventory file is
updated every time the provisioner runs unless a file is provided.
Therefore if Ansible attempts to provision two nodes in parallel, you
may see the following race:
* System A writes the inventory file and calls Ansible.
* System B starts to provision and truncates the file before
creating a new one.
* Ansible on system A now attempts to read the inventory
file, which is blank. Ansible bombs out with "ERROR: provided
hosts list is empty".
To fix this, we only allow Vagrant to update the inventory file if
it needs to.
eb6aa2ac8c