Add flexible IP pool/address handling.

This is refactor of floating IP support to keep it within the scope of the OpenStack plugin. Adds the following template fields:

"use_floating_ip": true    // Tells packer to use a temporary floating ip from the default pool "public" for SSH
"floating_ip_pool": "pool-name"    // Tells packer to use a temporary floating ip from "pool-name" for SSH
"floating_ip": "ip_address"    // Tells packer to use "ip_address" for SSH access to the instance

Depends on: https://github.com/rackspace/gophercloud/pull/139

Some other go fmt fixes have made their way in here as well.