Add write tests docs for using client managers and credentials

This commit adds documentation on how to interact with clients, client
managers, and credentials objects. These are critical pieces of
information for anyone writing tempest tests, so making sure the guide
includes this information is equally important.

Change-Id: I89e2c137068eea8aeaff0561fe078eccd3a23f0e

doc/source/write_tests.rst

@@ -112,6 +112,7 @@ of these would be::
         super(TestExampleCase, cls).resource_setup()
         cls.shared_server = cls.servers_client.create_server(...)
 
+.. _credentials:
 
 Allocating Credentials
 ''''''''''''''''''''''
@@ -241,3 +242,70 @@ different services (and scenario tests) to override this setting. When
 inheriting from classes other than the base TestCase in tempest/test.py it is
 worth checking the immediate parent for what is set to determine if your
 class needs to override that setting.
+
+Interacting with Credentials and Clients
+========================================
+
+Once you have your basic TestCase setup you'll want to start writing tests. To
+do that you need to interact with an OpenStack deployment. This section will
+cover how credentials and clients are used inside of Tempest tests.
+
+
+Manager Objects
+---------------
+
+The primary interface with which you interact with both credentials and
+API clients is the client manager object. These objects are created
+automatically by the base test class as part of credential setup. (for more
+details see the previous :ref:`credentials` section) Each manager object is
+initialized with a set of credentials and has each client object already setup
+to use that set of credentials for making all the API requests. Each client is
+accessible as a top level attribute on the manager object. So to start making
+API requests you just access the client's method for making that call and the
+credentials are already setup for you. For example if you wanted to make an API
+call to create a server in Nova::
+
+  from tempest import test
+
+
+  class TestExampleCase(test.BaseTestCase):
+    def test_example_create_server(self):
+      self.os.servers_client.create_server(...)
+
+is all you need to do. As described previously, in the above example the ``self.os``
+is created automatically because the base test class sets the ``credentials``
+attribute to allocate a primary credential set and initializes the client
+manager as ``self.os``. This same access pattern can be used for all of the
+clients in Tempest.
+
+Credentials Objects
+-------------------
+
+In certain cases you need direct access to the credentials. (the most common
+use case would be an API request that takes a user or project id in the request
+body) If you're in a situation where you need to access this you'll need to
+access the ``credentials`` object which is allocated from the configured
+credential provider in the base test class. This is accessible from the manager
+object via the manager's ``credentials`` attribute. For example::
+
+  from tempest import test
+
+
+  class TestExampleCase(test.BaseTestCase):
+    def test_example_create_server(self):
+      credentials = self.os.credentials
+
+The credentials object provides access to all of the credential information you
+would need to make API requests. For example, building off the previous
+example::
+
+  from tempest import test
+
+
+  class TestExampleCase(test.BaseTestCase):
+    def test_example_create_server(self):
+      credentials = self.os.credentials
+      username = credentials.username
+      user_id = credentials.user_id
+      password = credentials.password
+      tenant_id = credentials.tenant_id