Get credential IDs from Credentials class

With the move to Credentials class, tenant_id and user_id are
available in the Credentials instance.
Several test use identity admin client to obtain tenant_id
and user_id, fixing them to get the IDs from Credentials instead.

Partially implements: bp multi-keystone-api-version-tests

Change-Id: I8c38fb8d191a88fab37633c6edffd54492c41fd5
diff --git a/tempest/api/compute/admin/test_servers_negative.py b/tempest/api/compute/admin/test_servers_negative.py
index 9fa07f6..8b3a0b5 100644
--- a/tempest/api/compute/admin/test_servers_negative.py
+++ b/tempest/api/compute/admin/test_servers_negative.py
@@ -37,10 +37,7 @@
         cls.client = cls.os_adm.servers_client
         cls.non_adm_client = cls.servers_client
         cls.flavors_client = cls.os_adm.flavors_client
-        cls.identity_client = cls._get_identity_admin_client()
-        tenant = cls.identity_client.get_tenant_by_name(
-            cls.client.tenant_name)
-        cls.tenant_id = tenant['id']
+        cls.tenant_id = cls.client.tenant_id
 
         cls.s1_name = data_utils.rand_name('server')
         resp, server = cls.create_test_server(name=cls.s1_name,