Change ansible version to 2.4.2

Change ansible version to 2.4.2

Modified requirements.txt to update ansible version to 2.4.2

For the ansible virt module [1] XML templates are required.

To edit, the XML templates the XML module is required.
This is new in ansible 2.4 [2]

There is no max version restriction for ansible in OpenStack [3].

[1] http://docs.ansible.com/ansible/latest/virt_module.html
[2] https://docs.ansible.com/ansible/2.4/xml_module.html
[3] https://github.com/openstack/kolla-ansible/blob/stable/pike/doc/source/user/quickstart.rst

JIRA: YARDSTICK-948

Change-Id: I19b178f695f7381a3ecfbdb74f953a1276250958
Signed-off-by: John O Loughlin <john.oloughlin@intel.com>