Disable SSH host key checking for Ansible.

Created by: smarnach

Follow-up to #300.

The Ansible configuration in edx/configuration disables SSH host key checking. The first time a playbook runs for a given IP, SSH will accept the host key unconditionally and add it to ~/.ssh/known_hosts. The ansible.cfg in ansible-playbooks, by contrast, does not disable host key checking. Most of the time this isn't a problem, since by the time the playbook runs the host key is already in known_hosts. If we by chance get assigned an IP that we already had for an earlier app server, we will have a different host key in known_hosts. Ansible will ignore the discrepancy for the upstream playbook, since host key checking is disabled, but SSH won't add the key to known_hosts, because there already is a different key for the same IP. In this case, our playbook will fail.

This PR disables host key checking for all Ansible runs.