Fix testing for the write-devstack-local-conf role

The test_plugin_deps function in the test code for the
write-devstack-local-conf role was missing the import part of actually
executing the code under test and asserting the expected result.

Change-Id: I125870b13d2581cdec0dede11157b19b702565cd
diff --git a/roles/write-devstack-local-conf/library/test.py b/roles/write-devstack-local-conf/library/test.py
index 791552d..65d327b 100644
--- a/roles/write-devstack-local-conf/library/test.py
+++ b/roles/write-devstack-local-conf/library/test.py
@@ -104,6 +104,22 @@
                  plugins=plugins,
                  base_dir=self.tmpdir,
                  path=os.path.join(self.tmpdir, 'test.local.conf'))
+        lc = LocalConf(p.get('localrc'),
+                       p.get('local_conf'),
+                       p.get('base_services'),
+                       p.get('services'),
+                       p.get('plugins'),
+                       p.get('base_dir'),
+                       p.get('projects'),
+                       p.get('project'))
+        lc.write(p['path'])
+
+        plugins = []
+        with open(p['path']) as f:
+            for line in f:
+                if line.startswith('enable_plugin'):
+                    plugins.append(line.split()[1])
+        self.assertEqual(['foo', 'bar'], plugins)
 
     def test_libs_from_git(self):
         "Test that LIBS_FROM_GIT is auto-generated"