Fix #923, support image with ids instead of names.

Signed-off-by: Daniel Nephin <dnephin@gmail.com>

tests/integration/testcases.py

@@ -22,7 +22,7 @@ class DockerClientTestCase(unittest.TestCase):
                 self.client.remove_image(i)
 
     def create_service(self, name, **kwargs):
-        kwargs['image'] = "busybox:latest"
+        kwargs['image'] = kwargs.pop('image', 'busybox:latest')
 
         if 'command' not in kwargs:
             kwargs['command'] = ["/bin/sleep", "300"]