Like Vagrant's default SSH behaviors (e.g ssh or ssh-config commands),
the Ansible provisioner should by default not modify or read the user
known host file (e.g. ~/.ssh/known_hosts).
Given that `UserKnownHostsFile=/dev/null` SSH option is usually combined
with `StrictHostKeyChecking=no`, it seems quite reasonable to bind the
activation/disactivation of both options to `host_key_checking`
For the records, a discussion held in Ansible-Development mailing list
clearly confirmed that there is no short-term plan to adapt Ansible to
offer an extra option or change the behavior of
ANSIBLE_HOST_KEY_CHECKING. For this reason, the current implementation
seems reasonable and should be stable on the long run.