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: I3681b1a322faf7226236fb4f5db8b0308f8fd9b9

tox.ini

@@ -116,7 +116,7 @@ setenv =
     ANSIBLE_LIBRARY = {homedir}/.ansible/roles/plugins/library
     ANSIBLE_ROLES_PATH = {homedir}/.ansible/roles
 commands =
-    rm -rf {homedir}/.ansible
+    rm -rf {homedir}/.ansible/roles
     ansible-galaxy install \
                    --role-file={toxinidir}/ansible-role-requirements.yml \
                    --force