api: Add response body schemas for quota class sets API

We replace the use of 'test_class' in a variety of places with
'default', since nova only supports the 'default' quota class set. We
also start using the quota fixture in the policy tests to ensure we
return "valid" responses.

Change-Id: I6df0ec741f355e71c359c4e94bb32caacf9b1fab
Signed-off-by: Stephen Finucane <stephenfin@redhat.com>

doc/api_samples/os-quota-class-sets/quota-classes-show-get-resp.json

@@ -3,7 +3,7 @@
         "cores": 20,
         "fixed_ips": -1,
         "floating_ips": -1,
-        "id": "test_class",
+        "id": "default",
         "injected_file_content_bytes": 10240,
         "injected_file_path_bytes": 255,
         "injected_files": 5,

doc/api_samples/os-quota-class-sets/v2.50/quota-classes-show-get-resp.json

@@ -1,7 +1,7 @@
 {
     "quota_class_set": {
         "cores": 20,
-        "id": "test_class",
+        "id": "default",
         "injected_file_content_bytes": 10240,
         "injected_file_path_bytes": 255,
         "injected_files": 5,

doc/api_samples/os-quota-class-sets/v2.57/quota-classes-show-get-resp.json

@@ -1,7 +1,7 @@
 {
     "quota_class_set": {
         "cores": 20,
-        "id": "test_class",
+        "id": "default",
         "instances": 10,
         "key_pairs": 100,
         "metadata_items": 128,