Install SNAT rules for ipv4 only

Change-Id: I37bd770aa0d54a985ac2bec708c571785084e0ec
Closes-Bug: #1309195
(cherry picked from commit d23bc8fa6e2d8a735a2aa75224b1bc96a3b992f5)
Merge branch 'stable/havana' of https://github.com/openstack/quantum into havana/stable