Add private ip to /etc/nodepool

The private IPs are more useful for having nodes to
 communicate with each other in multi-node jobs.

This change extends the /etc/nodepool with two new files.
* primary_node_private The private ip of the primary node
* sub_nodes_private The private ip of the sub nodes,
  in the same order as the floating ones.

This change helps in simplify the devstack-gate multi-node change.
This change causes DB schema change, both node and subnode table
extended with an ip_private field.

Change-Id: Ib7b29afbfae877e6edeea6568732c54c3d9e12f1