It may be necessary to install extra Python packages inside the
kolla-ansible virtualenv, such as when required by Ansible plugins. For
example, using the hashi_vault lookup plugin requires the hvac Python
package to be installed.

This patch adds a kolla_ansible_venv_extra_requirements list variable
which contains Python package requirements to add to the kolla-ansible
requirements file. Its default value is an empty list.

Change-Id: Ie0541dc05e2ab94af230d0113a23c544755424c7