Add default value for CONF.auth.admin_domain_name

It's a bit confusing for the users who are not so familiar
with keystone v3 to configure the value of CONF.auth.admin_domain_name,
so better to provide a default value for this config item,
which can work in most environments.

Change-Id: I21705f6c4966e507e31201e3efe6d0d593526020

tempest/config.py

@@ -104,6 +104,7 @@ AuthGroup = [
                secret=True,
                deprecated_group='identity'),
     cfg.StrOpt('admin_domain_name',
+               default='Default',
                help="Admin domain name for authentication (Keystone V3)."
                     "The same domain applies to user and project",
                deprecated_group='identity'),