test.py: stop using aliases for creds manager

It could be confusing to new comers that the variables
cls.os and cls.os_primary or cls.alt_manager and cls.os_alt
actually are aliases. Besides we are not consistent in their usage.

This patch normalizes the usage around os_admin, os_primary and
os_alt. We keep the old aliases for compatibility but we should not
use them anymore.

This fix a long standing TODO item.

Change-Id: I5f7164f7a7ec5d4380ca22885000caa0183a0bf7
diff --git a/tempest/api/compute/admin/test_agents.py b/tempest/api/compute/admin/test_agents.py
index 4ae4372..31976ec 100644
--- a/tempest/api/compute/admin/test_agents.py
+++ b/tempest/api/compute/admin/test_agents.py
@@ -23,7 +23,7 @@
     @classmethod
     def setup_clients(cls):
         super(AgentsAdminTestJSON, cls).setup_clients()
-        cls.client = cls.os_adm.agents_client
+        cls.client = cls.os_admin.agents_client
 
     @classmethod
     def resource_setup(cls):