Fix floating_ip detection for OpenStack Folsom Release

OS-EXT-IPS:type was introduced only in Grizzly, thus Folsom
cannot rely on this api extension.
See https://bugs.launchpad.net/openstack-api-site/+bug/1128562
Meanwhile we can designate floating ip from fixed_ip by provided
fallback
Merge pull request #3211 from ytsarev/folsom_floating_ip

Fix floating_ip detection for OpenStack Folsom Release