Revert back to using [heat_plugin] section in tempest.conf

As we're now installing the plugin and running the tests in
a venv at the gate, there is no conflict.

Change-Id: I63dccc94d6954872257062b12114fab07a4e0873
diff --git a/heat_tempest_plugin/common/test.py b/heat_tempest_plugin/common/test.py
index 97d8d8c..a0a8b5f 100644
--- a/heat_tempest_plugin/common/test.py
+++ b/heat_tempest_plugin/common/test.py
@@ -79,7 +79,7 @@
 
     The decorated test will be skipped when convergence is disabled.
     '''
-    plugin = config.CONF.orchestration_plugin
+    plugin = config.CONF.heat_plugin
     convergence_enabled = plugin.convergence_engine_enabled
     skipper = testtools.skipUnless(convergence_enabled,
                                    "Convergence-only tests are disabled")
@@ -92,7 +92,7 @@
     def setUp(self):
         super(HeatIntegrationTest, self).setUp()
 
-        self.conf = config.CONF.orchestration_plugin
+        self.conf = config.CONF.heat_plugin
 
         self.assertIsNotNone(self.conf.auth_url,
                              'No auth_url configured')