Disable image validation

In before, Zun validate the container's image before lauching
the container. The goal is to have a pre-flight check before
doing an expensive operation.

However, this feature has several disadvantages:
* It blocks the zun api server on validating the image
* It relies on API call to external image registry (i.e. DockerHub),
  so it is not reliable.

This commit changes the default configuration to disable image
validation by default.

Change-Id: I67ad3fd69bc7680fb565bfc138bc0d8b77157649

zun/conf/api.py

@@ -41,7 +41,7 @@ api_service_opts = [
                default="api-paste.ini",
                help="Configuration file for WSGI definition of API."),
     cfg.BoolOpt('enable_image_validation',
-                default=True,
+                default=False,
                 help="Enable image validation.")
 ]
 

zun/tests/unit/compute/test_compute_api.py

@@ -45,6 +45,7 @@ class TestAPI(base.TestCase):
                               mock_image_search,
                               mock_container_create,
                               mock_record_action_start):
+        CONF.set_override('enable_image_validation', True, group="api")
         container = self.container
         container.status = consts.CREATING
         image_meta = mock.MagicMock()
@@ -66,6 +67,7 @@ class TestAPI(base.TestCase):
     def test_container_create_with_private_registry_image(
             self, mock_schedule_container, mock_image_search,
             mock_container_create, mock_record_action_start):
+        CONF.set_override('enable_image_validation', True, group="api")
         container = self.container
         container.image = 'myregistry.io/test-image'
         container.image_driver = 'docker'
@@ -100,6 +102,7 @@ class TestAPI(base.TestCase):
     def test_searching_image_exception(self, mock_save,
                                        mock_schedule_container,
                                        mock_image_search):
+        CONF.set_override('enable_image_validation', True, group="api")
         container = self.container
         container.status = consts.CREATING
         mock_schedule_container.return_value = {'host': u'Centos',