Add test coverage of existing server external events policies

Current tests do not have good test coverage of existing policies.
Either tests for policies do not exist or if they exist then they
do not cover the actual negative and positive testing.

For Example, if any policy with default rule as admin only then
test should verify:
- policy check pass with context having admin or server owner
- policy check fail with context having non-admin and not server owner

As discussed in policy-defaults-refresh, to change the policies
with new default roles and scope_type, we need to have the enough
testing coverage of existing policy behavior.

When we will add the scope_type in policies or new default roles,
then these test coverage will be extended to adopt the new changes
and also make sure we do not break the existing behavior.

This commit covers the testing coverage of existing server
external events policies.

Partial implement blueprint policy-defaults-refresh

Change-Id: I58f6c04b0525526c66615e6b03cee868abbba442

nova/tests/unit/policies/test_server_external_events.py

@@ -0,0 +1,79 @@
+#    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.
+
+import mock
+from oslo_utils.fixture import uuidsentinel as uuids
+
+from nova.api.openstack.compute import server_external_events as ev
+from nova.policies import server_external_events as policies
+from nova.tests.unit.api.openstack import fakes
+from nova.tests.unit.policies import base
+
+
+class ServerExternalEventsPolicyTest(base.BasePolicyTest):
+    """Test Server External Events APIs policies with all possible context.
+
+    This class defines the set of context with different roles
+    which are allowed and not allowed to pass the policy checks.
+    With those set of context, it will call the API operation and
+    verify the expected behaviour.
+    """
+
+    def setUp(self):
+        super(ServerExternalEventsPolicyTest, self).setUp()
+        self.controller = ev.ServerExternalEventsController()
+        self.req = fakes.HTTPRequest.blank('')
+
+        # Check that admin is able to create the server external events.
+        self.admin_authorized_contexts = [
+            self.legacy_admin_context, self.system_admin_context,
+            self.project_admin_context
+        ]
+        # Check that non-admin is not able to create the server
+        # external events.
+        self.admin_unauthorized_contexts = [
+            self.system_member_context, self.system_reader_context,
+            self.system_foo_context, self.project_member_context,
+            self.project_reader_context, self.project_foo_context,
+            self.other_project_member_context
+        ]
+
+    @mock.patch('nova.compute.api.API.external_instance_event')
+    @mock.patch('nova.objects.InstanceMappingList.get_by_instance_uuids')
+    @mock.patch('nova.objects.InstanceList.get_by_filters')
+    def test_server_external_events_policy(self, mock_event, mock_get,
+                                           mock_filter):
+        rule_name = policies.POLICY_ROOT % 'create'
+        body = {'events': [{'name': 'network-vif-plugged',
+                           'server_uuid': uuids.fake_id,
+                           'status': 'completed'}]
+               }
+        self.common_policy_check(self.admin_authorized_contexts,
+                                 self.admin_unauthorized_contexts,
+                                 rule_name, self.controller.create,
+                                 self.req, body=body)
+
+
+class ServerExternalEventsScopeTypePolicyTest(ServerExternalEventsPolicyTest):
+    """Test Server External Events APIs policies with system scope enabled.
+
+    This class set the nova.conf [oslo_policy] enforce_scope to True
+    so that we can switch on the scope checking on oslo policy side.
+    It defines the set of context with scoped token
+    which are allowed and not allowed to pass the policy checks.
+    With those set of context, it will run the API operation and
+    verify the expected behaviour.
+    """
+
+    def setUp(self):
+        super(ServerExternalEventsScopeTypePolicyTest, self).setUp()
+        self.flags(enforce_scope=True, group="oslo_policy")