Added Nodes list for Rack

This patch adds several things:

* Add 'Node' database model
* Add 1-m relation between Rack->Node
* Add 'Node' representation in controller
* Support creating Rack with list of initial Nodes

Example:

Create Rack with Hosts:

```bash
$ curl -X POST -H 'Content-Type: application/json' -H 'Accept: application/json' -v -d '{"subnet":"192.168.1.0/255","name":"my_rack","capacities":[{"name":"total_cpu","value":"64"},{"name":"total_memory","value":"1024"}],"nodes":{"links":[{"href":"http://ironic.local:3333/v1/node/123","rel":"node"},{"href":"http://ironic.local:3333/v1/node/456","rel":"node"}]},"slots":1}' http://0.0.0.0:6385/v1/racks
```

FYI, the uncompressed JSON looks like this:

```json
{
   "subnet":"192.168.1.0/255",
   "name":"my_rack",
   "capacities":[
      {
         "name":"total_cpu",
         "value":"64"
      },
      {
         "name":"total_memory",
         "value":"1024"
      }
   ],
   "nodes":{
      "links":[
         {
            "href":"http://ironic.local:3333/v1/node/123",
            "rel":"node"
         },
         {
            "href":"http://ironic.local:3333/v1/node/456",
            "rel":"node"
         }
      ]
   },
   "slots":1
}
```