Use plugins repo version of the human_log callback plugin

With the implementation of https://review.openstack.org/321331 the
human_log callback plugin is now part of the plugins repo.

This patch removes the retrofitted version in tox in favor of using
the version from the plugins repo instead.

Change-Id: I3c817b3019a892611d8dfbe0a1b3d3610857afdb

tox.ini

@@ -126,9 +126,6 @@ commands =
     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 \