Added Hosts list for Rack

This patch adds several things:

* Add 'Host' database model
* Add 1-m relation between Rack->Host
* Add 'Host' representation in controller
* Support creating Rack with list of initial Hosts

Example:

Create Rack with Hosts:

```bash
{
  "subnet": "192.168.1.0/255",
  "name": "my_rack",
  "capacities": [
    {
      "name": "total_cpu",
      "value": "64"
    },
    {
      "name": "total_memory",
      "value": "1024"
    }
  ],
  "hosts": {
    "links": [
      {
        "href": "http://ironic.local:3333/v1/node/123",
        "rel": "node"
      },
      {
        "href": "http://ironic.local:3333/v1/node/456",
        "rel": "node"
      }
    ]
  },
  "slots": 1,
}
```