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: I1aad3b4e4fecc9e2b030c50b09e83b2b57ab2eb3

openstack/openstack-ansible-rabbitmq_server,openstack/openstack-ansible-rabbitmq_server