Add API tests for config templates

Update the `ConfigTemplate` class definition: add mixins for creation, reading
and deletion. Thoroughly test the relevant API endpoints.
Should be able to fetch available template 'kinds' though, so perhaps at least make sure that we can use ``path(which='template_kinds'``?

```python
In [13]: client.get('https://SERVER/api/v2/template_kinds', auth=get_server_credentials(), verify=False).json()
2014-09-05 15:03:33 - robottelo.api.client - INFO - Making HTTP GET request to https://SERVER/api/v2/template_kinds with options {'verify': False, 'data': '{}', 'auth': ('admin', 'changeme'), 'headers': {'content-type': 'application/json'}} and no data.
2014-09-05 15:03:33 - robottelo.api.client - INFO - Equivalent curl command: curl -X GET --user admin:changeme --insecure data=%7B%7D&headers=%7B%27content-type%27%3A+%27application%2Fjson%27%7D https://SERVER/api/v2/template_kinds
2014-09-05 15:03:34 - requests.packages.urllib3.connectionpool - INFO - Starting new HTTPS connection (1): qe-sat6-rhel7.usersys.redhat.com
2014-09-05 15:03:35 - requests.packages.urllib3.connectionpool - DEBUG - "GET /api/v2/template_kinds HTTP/1.1" 200 None
2014-09-05 15:03:35 - robottelo.api.client - DEBUG - Received HTTP 200 response: {
  "total": 8,
  "subtotal": 8,
  "page": 1,
  "per_page": 1400,
  "search": null,
  "sort": {
    "by": null,
    "order": null
  },
  "results": [{"name":"PXELinux","id":1,"created_at":"2014-09-04T22:27:52Z","updated_at":"2014-09-04T22:27:52Z"},{"name":"PXEGrub","id":2,"created_at":"2014-09-04T22:27:52Z","updated_at":"2014-09-04T22:27:52Z"},{"name":"iPXE","id":3,"created_at":"2014-09-04T22:27:52Z","updated_at":"2014-09-04T22:27:52Z"},{"name":"provision","id":4,"created_at":"2014-09-04T22:27:52Z","updated_at":"2014-09-04T22:27:52Z"},{"name":"finish","id":5,"created_at":"2014-09-04T22:27:52Z","updated_at":"2014-09-04T22:27:52Z"},{"name":"script","id":6,"created_at":"2014-09-04T22:27:52Z","updated_at":"2014-09-04T22:27:52Z"},{"name":"user_data","id":7,"created_at":"2014-09-04T22:27:52Z","updated_at":"2014-09-04T22:27:52Z"},{"name":"ZTP","id":8,"created_at":"2014-09-04T22:27:52Z","updated_at":"2014-09-04T22:27:52Z"}]
}

Out[13]:
{u'page': 1,
 u'per_page': 1400,
 u'results': [{u'created_at': u'2014-09-04T22:27:52Z',
   u'id': 1,
   u'name': u'PXELinux',
   u'updated_at': u'2014-09-04T22:27:52Z'},
  {u'created_at': u'2014-09-04T22:27:52Z',
   u'id': 2,
   u'name': u'PXEGrub',
   u'updated_at': u'2014-09-04T22:27:52Z'},
  {u'created_at': u'2014-09-04T22:27:52Z',
   u'id': 3,
   u'name': u'iPXE',
   u'updated_at': u'2014-09-04T22:27:52Z'},
  {u'created_at': u'2014-09-04T22:27:52Z',
   u'id': 4,
   u'name': u'provision',
   u'updated_at': u'2014-09-04T22:27:52Z'},
  {u'created_at': u'2014-09-04T22:27:52Z',
   u'id': 5,
   u'name': u'finish',
   u'updated_at': u'2014-09-04T22:27:52Z'},
  {u'created_at': u'2014-09-04T22:27:52Z',
   u'id': 6,
   u'name': u'script',
   u'updated_at': u'2014-09-04T22:27:52Z'},
  {u'created_at': u'2014-09-04T22:27:52Z',
   u'id': 7,
   u'name': u'user_data',
   u'updated_at': u'2014-09-04T22:27:52Z'},
  {u'created_at': u'2014-09-04T22:27:52Z',
   u'id': 8,
   u'name': u'ZTP',
   u'updated_at': u'2014-09-04T22:27:52Z'}],
 u'search': None,
 u'sort': {u'by': None, u'order': None},
 u'subtotal': 8,
 u'total': 8}
```