Use plugins repo version of the human_log callback plugin
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: I7775cd6f46c0aad9ef7837f6a81556733bb4909a

os-cloud/openstack-ansible-pip_install,openstack/openstack-ansible-pip_install,os-cloud/openstack-ansible-pip_install,openstack/openstack-ansible-pip_install