Fix wrong detection of plugin deletable property

Add tests

Change-Id: I1577f2035777319219980ec181d8275597c5bced
Closes-Bug: #1632377
(cherry picked from commit f982baccb2)

nailgun/nailgun/plugins/manager.py

@@ -765,7 +765,7 @@ class PluginManager(object):
         :type plugin: plugin model
         :returns: boolean
         """
-        return ClusterPlugin.is_plugin_used(plugin.id)
+        return not ClusterPlugin.is_plugin_used(plugin.id)
 
     @classmethod
     def _get_specific_version(cls, versions, plugin_id):

nailgun/nailgun/test/unit/test_plugin_manager.py

@@ -0,0 +1,42 @@
+# -*- coding: utf-8 -*-
+
+#    Copyright 2017 Mirantis, Inc.
+#
+#    Licensed under the Apache License, Version 2.0 (the "License"); you may
+#    not use this file except in compliance with the License. You may obtain
+#    a copy of the License at
+#
+#         http://www.apache.org/licenses/LICENSE-2.0
+#
+#    Unless required by applicable law or agreed to in writing, software
+#    distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+#    WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+#    License for the specific language governing permissions and limitations
+#    under the License.
+
+from nailgun import objects
+from nailgun.plugins import manager
+
+from nailgun.test.base import BaseTestCase
+
+
+class TestPluginManager(BaseTestCase):
+
+    def setUp(self):
+        super(TestPluginManager, self).setUp()
+        self.cluster = self.env.create()
+        self.plugin_manager = manager.PluginManager
+        self.plugin = self.env.create_plugin(api=False)
+
+    def test_return_false_if_plugin_used(self):
+        self.cluster.plugins.append(self.plugin)
+        objects.ClusterPlugin.set_attributes(self.cluster.id,
+                                             self.plugin.id,
+                                             enabled=True)
+        self.db.refresh(self.plugin)
+        self.assertEqual(False, self.plugin_manager
+                         ._is_plugin_deletable(self.plugin))
+
+    def test_return_true_if_plugin_unused(self):
+        self.assertEqual(True, self.plugin_manager
+                         ._is_plugin_deletable(self.plugin))