Fix: Doc build by referring the Footnote
Fix: Doc build by referring the Footnote

Footnote for zabbix manual was defined but not referenced,
this causes Sphinx WARNING: Footnote [#] is not referenced
and fails doc build(tox -edocs) as Warning is treated as
Error is enabled by adding flag(-W). This patch fixes it.

Change-Id: If2a85ec55f8e5ee887c49c2f603c1357f7166f93

stackforge/tacker,openstack/tacker,stackforge/tacker,openstack/tacker,openstack/tacker