Fix test `test_cluster.TestNode.test_start` by adding keyword argument
`node_name` to the arguments of `start_instance()` call.
add option request_floating_ip for openstack cloud in configuration file validator
Remove attributes `ip_public` and `ip_private` from `cluster.Node`.
Add attributes `ips` (list of string) and `preferred_ip` (string).
Modify EC2 and OpenStack backends to find the best IP to be used for connection.
Modify `BotoCloudProvider` and `OpensStackCloudProvider` to return a list of all IPs of the instance
Modify `Cluster`, `AnsibleSetupProvider` and `SshFrontend` to use the `ips` and `preferred_ip` attributes of the `Node` class instead of the old attributes.