Add ansible-requirements as a constraint in CI

This change will ensure that the ansible-requirements.txt file is used
in molecule jobs as a constraint when creating a test environment.

Change-Id: I81b8201e17f8d2c0300f3518d6974e4ea2655ca0
Signed-off-by: Kevin Carter <kecarter@redhat.com>

zuul.d/playbooks/pre.yml

@@ -41,6 +41,8 @@
         virtualenv: "{{ ansible_user_dir }}/test-python"
         virtualenv_command: "{{ ensure_pip_virtualenv_command }}"
         virtualenv_site_packages: true
+        extra_args: >-
+          --constraint "{{ tripleo_ansible_project_path }}/ansible-requirements.txt"
 
     - name: Display test-python virtualenv package versions
       shell: |-