Remove OfficialClient dependency from HACKING.rst

This commit migrates OfficialClientTest to ScenarioTest about
TestVolumeBootPattern in HACKING.rst. We should not have such an example
in the HACKING.rst because we don't use OfficialClientTest class in the
scenario tests anymore.

Partially-implements: bp:tempest-client-scenarios

Change-Id: Id95d011f3d080effc7bfee6232578451f7ba72d9
diff --git a/HACKING.rst b/HACKING.rst
index 29d5bf4..83d67a9 100644
--- a/HACKING.rst
+++ b/HACKING.rst
@@ -259,7 +259,7 @@
 docstrings for the workflow in each test methods can be used instead. A good
 example of this would be::
 
-    class TestVolumeBootPattern(manager.OfficialClientTest):
+    class TestVolumeBootPattern(manager.ScenarioTest):
     """
     This test case attempts to reproduce the following steps: