Ensure IPPolicy response object exclude key has string cidr as value

 - currently returns IPPolicyCIDR object

example current response:

```javascript
{  
   "ip_policy":{  
      "name":null,
      "tenant_id":"5919851",
      "exclude":[  
         "<quark.db.models.IPPolicyCIDR[object at 3d59d90] 
              {id='0bb0ec24-d6b5-4249-9ebb-70083deb4d7a', 
               created_at=datetime.datetime(2014, 7, 30, 16, 35, 16, 216986), 
               ip_policy_id='af7c9607-0c74-4cd9-8e64-b7d3127c352c', 
               cidr=u'192.168.155.10/32'}>" 
      ],
      "subnet_ids":[  

      ],
      "id":"af7c9607-0c74-4cd9-8e64-b7d3127c352c",
      "network_ids":[  
         "69202465-b88e-4472-bcbf-03942e5e08cf" 
      ]
   }
}
```

what it will look like
```javascript
{  
   "ip_policy":{  
      "name":null,
      "tenant_id":"5919851",
      "exclude":[  
         "192.168.155.10/32"
      ],
      "subnet_ids":[  

      ],
      "id":"af7c9607-0c74-4cd9-8e64-b7d3127c352c",
      "network_ids":[  
         "69202465-b88e-4472-bcbf-03942e5e08cf" 
      ]
   }
}
```
 RM3793