Bump ansible_version_min to 2.4

the include_tasks is the new feature in version 2.4[0], and
kolla use it[1], the ansible_version_min should be updated
to version 2.4

[0]: https://docs.ansible.com/ansible/2.4/include_tasks_module.html
[1]: https://review.openstack.org/#/c/584728/

Change-Id: I273ac9e8295bfe3fb97af620c1e48e9fbbf06c6e

ansible/roles/prechecks/vars/main.yml

 ---
 docker_version_min: '1.10.0'
 docker_py_version_min: '2.0.0'
-ansible_version_min: '2.2.0'
+ansible_version_min: '2.4.0'