Ansible, Vagrant, and changed host keys

Related to this bug, the Vagrant Ansible provisioner seems to ignore some system settings.

The symptom is that when you update a previously used Vagrant box, or otherwise change its host key, Ansible provisioning fails with the error:

fatal: [hostname] => SSH Error: Host key verification failed.
    while connecting to
It is sometimes useful to re-run the command using -vvvv, which prints SSH
debug output to help diagnose the issue.

The standard solution would be to forget about the old host key with ssh-keygen -R or ignore the change with export ANSIBLE_HOST_KEY_CHECKING=false.

If you trust the box not to be evil and expect its host key to change frequently due to your testing, a fix which the Ansible provisioner does respect is to add ansible.host_key_checking = false to the Vagrantfile, like:

Vagrant.configure(2) do |config|
    config.vm.define "hostname" do |prodmaster|
        hostname.vm.provision "ansible" do |ansible|
            ansible.playbook = "provision/hostname.yaml"
            ansible.sudo = true
            ansible.host_key_checking = false
            ansible.verbose = 'vvvv'
            ansible.extra_vars = { ansible_ssh_user: 'vagrant'}