Support attaching FIPs to login nodes

[sjpb]

Allows attaching FIPs to login nodes, e.g.:

login = {
        login: {
            nodes: ["login-0"]
            flavor: var.other_node_flavor
            fip_addresses = ["45.135.56.139"]
        }
    }

Note FIPs must already be allocated to the project using e.g.:

openstack floating ip add external

This approach means cluster deletion does not release the FIPs from the project.

[jovial]

Tested and working for me