Remove default None value for config options

In oslo.config default is always initialized to None.

Change-Id: I4c8eb7eb43121d28364f6e2e3a2fbabcc1bdb90d
diff --git a/common/config.py b/common/config.py
index b45a8d8..c715c88 100644
--- a/common/config.py
+++ b/common/config.py
@@ -44,7 +44,6 @@
     cfg.StrOpt('image_ref',
                help="Name of image to use for tests which boot servers."),
     cfg.StrOpt('keypair_name',
-               default=None,
                help="Name of existing keypair to launch servers with."),
     cfg.StrOpt('minimal_image_ref',
                help="Name of minimal (e.g cirros) image to use when "