Use 'OS-EXT-SRV-ATTR:host' directly

On the history, Tempest needed to support Nova v2 and v3 APIs which
are different request/response parameters. And Tempest switched the
parameter names by using class values. However, Nova team has dropped
Nova v3 API and the corresponding tests also have been removed from
Tempest. Now the class values are unuseful and less readability.

This patch makes Tempest use 'OS-EXT-SRV-ATTR:host' directly.

Change-Id: Ia5d8d45578854279c914613d1dd5590e3cc0c60b
diff --git a/tempest/api/compute/admin/test_aggregates.py b/tempest/api/compute/admin/test_aggregates.py
index ac1bfee..fbcc1d1 100644
--- a/tempest/api/compute/admin/test_aggregates.py
+++ b/tempest/api/compute/admin/test_aggregates.py
@@ -25,8 +25,6 @@
 class AggregatesAdminTestJSON(base.BaseV2ComputeAdminTest):
     """Tests Aggregates API that require admin privileges"""
 
-    _host_key = 'OS-EXT-SRV-ATTR:host'
-
     @classmethod
     def setup_clients(cls):
         super(AggregatesAdminTestJSON, cls).setup_clients()
@@ -223,4 +221,4 @@
                                          availability_zone=az_name,
                                          wait_until='ACTIVE')
         body = admin_servers_client.show_server(server['id'])['server']
-        self.assertEqual(self.host, body[self._host_key])
+        self.assertEqual(self.host, body['OS-EXT-SRV-ATTR:host'])