Add node id to cloud load balancer's Node's to_dict method

Add a Cloud Load Balancer's Node's id to the to_dict method, for use by applications consuming it (e.g. Ansible's rax_clb module:  https://github.com/ansible/ansible/blob/devel/library/cloud/rax_clb).

More detail:
Ansible's rax_clb_nodes module currently requires node_id in order to work with nodes assigned to a load balancer.  However when the rax_clb module invokes pyrax to list existing nodes, node_id is not returned in the response.

While rax_clb will be adding the ability to match IPs directly, including node_id in the pyrax Cloud Load Balancer "Node" dict might be a good idea in general.