Remove default=None for configuration bindings

The default value for a configuration variable is None. There is
no need to specifically set these as None.

The line where this is done is:
https://github.com/openstack/oslo.config/blob/master/oslo_config/cfg.py#L684

TrivialFix

Change-Id: Iedf29b13d69ea9f2eb4d5aa746d7c1da8d83bafc
diff --git a/neutron/tests/tempest/config.py b/neutron/tests/tempest/config.py
index 200b247..256d687 100644
--- a/neutron/tests/tempest/config.py
+++ b/neutron/tests/tempest/config.py
@@ -73,7 +73,6 @@
                 default=False,
                 help="Set to True if using self-signed SSL certificates."),
     cfg.StrOpt('ca_certificates_file',
-               default=None,
                help='Specify a CA bundle file to use in verifying a '
                     'TLS (https) server certificate.'),
     cfg.StrOpt('uri',