Deprecate credential config options

This commit deprecates the original configuration options for
providing credentials. This mechanism of providing tempest with
credentials to run with has a lot of drawbacks and has been
superseded by using an accounts.yaml file. It has been planned
for deprecation once we had a periodic/experimental CI job setup
to run with test accounts enabled. Now that this has been done
we can start the deprecation cycle for the old options.

Change-Id: Ia5132c5cb32355d6f26b8acdd92a0e55a2c19f41
diff --git a/tempest/tests/fake_config.py b/tempest/tests/fake_config.py
index 4898c9c..ca8bc3e 100644
--- a/tempest/tests/fake_config.py
+++ b/tempest/tests/fake_config.py
@@ -48,9 +48,13 @@
         for config_option in ['username', 'password', 'tenant_name']:
             # Identity group items
             for prefix in ['', 'alt_', 'admin_']:
+                if prefix == 'admin_':
+                    group = 'auth'
+                else:
+                    group = 'identity'
                 self.conf.set_default(prefix + config_option,
                                       'fake_' + config_option,
-                                      group='identity')
+                                      group=group)
 
 
 class FakePrivate(config.TempestConfigPrivate):