Add server groups quota attributes in limit schema

If 'os-server-group-quotas' extension is enabled then,
absolute_limits GET API's response includes 'maxServerGroupMembers',
'maxServerGroups' and 'totalServerGroupsUsed' attributes too.

This patch adds those attributes in schema files.

Change-Id: I0d7adbdfe295d6cd27a8cb944fbaee3681113c42
diff --git a/tempest/api_schema/response/compute/v2/limits.py b/tempest/api_schema/response/compute/v2/limits.py
index b9857f1..a7decb7 100644
--- a/tempest/api_schema/response/compute/v2/limits.py
+++ b/tempest/api_schema/response/compute/v2/limits.py
@@ -38,8 +38,15 @@
                             'maxSecurityGroupRules': {'type': 'integer'},
                             'maxTotalKeypairs': {'type': 'integer'},
                             'totalRAMUsed': {'type': 'integer'},
-                            'totalInstancesUsed': {'type': 'integer'}
+                            'totalInstancesUsed': {'type': 'integer'},
+                            'maxServerGroupMembers': {'type': 'integer'},
+                            'maxServerGroups': {'type': 'integer'},
+                            'totalServerGroupsUsed': {'type': 'integer'}
                         },
+                        # NOTE(gmann): maxServerGroupMembers,  maxServerGroups
+                        # and totalServerGroupsUsed are API extension,
+                        # and some environments return a response without these
+                        # attributes.So they are not 'required'.
                         'required': ['maxImageMeta',
                                      'maxPersonality',
                                      'maxPersonalitySize',