Expose get_assigned_tos as virtual attribute to allow access via the API

[jntullo]

Follow up to ManageIQ/manageiq-api#348

This exposes get_assigned_tos as a virtual attribute so that the assigned objects and tags (classifications) can be returned like so:

{  
   "href":"http://localhost:3000/api/alert_definition_profiles/10000000000084",
   "id":"10000000000084",
   ...
   "get_assigned_tos":{  
      "objects":[  
         {  
            "href":"http://localhost:3000/api/vms/10000000000001",
            "id":"10000000000001",
            "vendor":"vmware",
            "format":null,
            "version":null,
            "name":"jmarc-nginx",
            "description":null,
            ...
         }
      ],
      "tags":[  
         [  
            {  
               "id":"10000000000198",
               "description":"api",
               "icon":null,
               "read_only":false,
              ...
            },
            "vms"
         ]
      ],
      "labels":[  

      ]
   }
}

Have validated (as can be seen above) that hrefs are returned for objects. These tags are "classification tags" (class Classification), which are not currently exposed via the API. Will have to follow up with a PR for Classification to return those hrefs

@miq-bot assign @abellotti

[miq-bot]

Checked commit jntullo@d7b6caf with ruby 2.3.3, rubocop 0.52.1, haml-lint 0.20.0, and yamllint 1.10.0
2 files checked, 0 offenses detected
Everything looks fine. 👍

[jntullo]

added test because once this exposed and available in API, would not want format to change

[abellotti]

Thanks @jntullo for this enhancement. 👍