Ensure that gate test does not remove ~/.ansible/tmp

As zuul uses Ansible the ~/.ansible directory is populated with
artifacts which zuul uses to facilitate the job execution.

This patch ensures that the execution of tests does not remove
the whole ~/.ansible directory, but instead only removes the
parts which we populate.

Change-Id: I299f19cd999869dd3f41d24fbdad044816998866

tox.ini

@@ -117,12 +117,13 @@ setenv =
     # specified in ansible.cfg
     ANSIBLE_ROLES_PATH = {homedir}/.ansible/roles:{toxinidir}/..
 commands =
-    rm -rf {homedir}/.ansible
+    rm -rf {homedir}/.ansible/plugins
     git clone https://git.openstack.org/openstack/openstack-ansible-plugins \
               {homedir}/.ansible/plugins
     # This plugin makes the ansible-playbook output easier to read
     wget -O {homedir}/.ansible/plugins/callback/human_log.py \
       https://gist.githubusercontent.com/cliffano/9868180/raw/f360f306b3c6d689734a6aa8773a00edf16a0054/human_log.py
+    rm -rf {homedir}/.ansible/roles
     ansible-galaxy install \
                    --role-file={toxinidir}/tests/ansible-role-requirements.yml \
                    --force