Allow replacing service implementation

Some clouds are offering their own incompatible implementation of system 
services. While with add_service we can add new implementation of the 
service, we can not replace system-ones. So let's allow deleting service 
to be able to have a custom service implementation in SDK.

Change-Id: I55ef310d114702e5dc98a237fc1d8d130c09c5e1

openstack/service_description.py

@@ -202,4 +202,13 @@ class ServiceDescription(object):
         raise AttributeError('Service Descriptors cannot be set')
 
     def __delete__(self, instance):
-        raise AttributeError('Service Descriptors cannot be deleted')
+        # NOTE(gtema) Some clouds are not very fast (or interested at all)
+        # in bringing their changes upstream. If there are incompatible changes
+        # downstream we need to allow overriding default implementation by
+        # deleting service_type attribute of the connection and then
+        # "add_service" with new implementation.
+        # This is implemented explicitely not very comfortable to use
+        # to show how bad it is not to contribute changes back
+        for service_type in self.all_types:
+            if service_type in instance._proxies:
+                del instance._proxies[service_type]

openstack/tests/unit/test_connection.py

@@ -308,3 +308,29 @@ class TestNewService(base.TestCase):
             'openstack.tests.unit.fake.v2._proxy',
             conn.fake.__class__.__module__)
         self.assertFalse(conn.fake.dummy())
+
+    def test_replace_system_service(self):
+        self.use_keystone_v3(catalog='catalog-v3-fake-v2.json')
+        conn = self.cloud
+
+        # delete native dns service
+        delattr(conn, 'dns')
+
+        self.register_uris([
+            dict(method='GET',
+                 uri='https://fake.example.com',
+                 status_code=404),
+            dict(method='GET',
+                 uri='https://fake.example.com/v2/',
+                 status_code=404),
+            dict(method='GET',
+                 uri=self.get_mock_url('fake'),
+                 status_code=404),
+        ])
+
+        # add fake service with alias 'DNS'
+        service = fake_service.FakeService('fake', aliases=['dns'])
+        conn.add_service(service)
+
+        # ensure dns service responds as we expect from replacement
+        self.assertFalse(conn.dns.dummy())