Introduce site_facts module

Because of a recent change to Ansible (in 1.6.7) the template var hack
that we were using no longer works. The site_facts module will now set
those facts. The tempest iptables role was also reworked to not need the
tempest_*_ip facts to be set.

Change-Id: Ia3e5b8c861cb1925dd4d30a9d909a4b99cc0d48c