TrivialFix: Remove default=None when set value in Config

By default oslo.cfg sets the default values as None [1], There is
no need to explicitly do this.

[1] https://github.com/openstack/oslo.config/blob/master/oslo_config/cfg.py#L829

Change-Id: Ibb74f1fedd1c20d97846308f30cb17dbcc1f6aba

barbican/plugin/interface/secret_store.py

@@ -50,7 +50,6 @@ store_opts = [
                          ' backend support. Default is False')
                 ),
     cfg.ListOpt('stores_lookup_suffix',
-                default=None,
                 help=u._('List of suffix to use for looking up plugins which '
                          'are supported with multiple backend support.')
                 )

barbican/plugin/util/multiple_backends.py

@@ -51,12 +51,10 @@ def read_multiple_backends_config():
                 name=group_name, title='Plugins needed for this backend')
             store_opts = [
                 cfg.StrOpt('secret_store_plugin',
-                           default=None,
                            help=u._('Internal name used to identify'
                                     'secretstore_plugin')
                            ),
                 cfg.StrOpt('crypto_plugin',
-                           default=None,
                            help=u._('Internal name used to identify '
                                     'crypto_plugin.')
                            ),