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/doc/source/write_tests.rst b/doc/source/write_tests.rst
index 8488fb1..2363fa6 100644
--- a/doc/source/write_tests.rst
+++ b/doc/source/write_tests.rst
@@ -99,7 +99,7 @@
            specific situations you should not need to use this.
         """
         super(TestExampleCase, cls).setup_clients()
-        cls.servers_client = cls.os.servers_client
+        cls.servers_client = cls.os_primary.servers_client
 
     @classmethod
     def resource_setup(cls):
@@ -143,10 +143,10 @@
 
 In this example the ``TestExampleAdmin`` TestCase will allocate 2 sets of
 credentials, one regular user and one admin user. The corresponding manager
-objects will be set as class variables cls.os and cls.os_adm respectively. You
-can also allocate a second user by putting **'alt'** in the list too. A set of
-alt credentials are the same as primary but can be used for tests cases that
-need a second user/project.
+objects will be set as class variables ``cls.os_primary`` and ``cls.os_admin``
+respectively. You can also allocate a second user by putting **'alt'** in the
+list too. A set of alt credentials are the same as primary but can be used
+for tests cases that need a second user/project.
 
 You can also specify credentials with specific roles assigned. This is useful
 for cases where there are specific RBAC requirements hard coded into an API.