The ansible docs have a section on this. Quoting:
Ansible has host key checking enabled by default.
If a host is reinstalled and has a different key in ‘known_hosts’,
this will result in an error message until corrected. If a host is not
initially in ‘known_hosts’ this will result in prompting for
confirmation of the key, which results in an interactive experience if
using Ansible, from say, cron. You might not want this.If you understand the implications and wish to disable this behavior,
you can do so by editing /etc/ansible/ansible.cfg or ~/.ansible.cfg:
[defaults]
host_key_checking = False
Alternatively this can be set by the
ANSIBLE_HOST_KEY_CHECKING
environment variable:
$ export ANSIBLE_HOST_KEY_CHECKING=False
Also note that host key checking in paramiko mode is reasonably slow, therefore switching to ‘ssh’ is also recommended when using this feature.