Introduce a config for default image driver

This allows caller to load an image driver without specifying
a specific driver. If the image driver is not provided by caller,
the default image driver will be loaded.

Change-Id: Iab1fc6ffa8093d74354a541a9900201b24120456

zun/conf/image_driver.py

@@ -27,7 +27,11 @@ Services which consume this:
 * ``zun-compute``
 Interdependencies to other options:
 * None
-""")
+"""),
+    cfg.StrOpt(
+        'default_image_driver',
+        default='docker',
+        help='The default container image driver to use.'),
 ]
 
 sandbox_opts = [

zun/image/driver.py

@@ -35,6 +35,8 @@ def load_image_driver(image_driver=None):
     :param image_driver: container image driver name to override config opt
     :returns: a ContainerImageDriver instance
     """
+    if not image_driver:
+        image_driver = CONF.default_image_driver
     if not image_driver:
         LOG.error("Container image driver option required, "
                   "but not specified")

zun/tests/unit/image/test_driver.py

@@ -13,7 +13,9 @@
 # under the License.
 
 import zun.conf
+from zun.image.docker import driver as docker_driver
 from zun.image import driver
+from zun.image.glance import driver as glance_driver
 from zun.tests import base
 
 CONF = zun.conf.CONF
@@ -24,10 +26,15 @@ class TestDriver(base.BaseTestCase):
         super(TestDriver, self).setUp()
 
     def test_load_image_driver_failure(self):
+        CONF.set_override('default_image_driver', None)
         self.assertRaises(SystemExit, driver.load_image_driver)
         self.assertRaises(SystemExit, driver.load_image_driver,
                           'UnknownDriver')
 
     def test_load_image_driver(self):
+        image_driver = driver.load_image_driver()
+        self.assertIsInstance(image_driver, docker_driver.DockerDriver)
+
         CONF.set_override('images_directory', None, group='glance')
-        self.assertTrue(driver.load_image_driver, 'glance.GlanceDriver')
+        image_driver = driver.load_image_driver('glance')
+        self.assertIsInstance(image_driver, glance_driver.GlanceDriver)