No need for 'default=None' in config variable

By default the value is None. There is no need to specify
this.

Change-Id: Iffb9dcaad366330e1bad3a3a0e707bb833953c08

ironic/api/__init__.py

@@ -29,7 +29,6 @@ API_SERVICE_OPTS = [
                help=_('The maximum number of items returned in a single '
                       'response from a collection resource.')),
     cfg.StrOpt('public_endpoint',
-               default=None,
                help=_("Public URL to use when building the links to the API "
                       "resources (for example, \"https://ironic.rocks:6384\")."
                       " If None the links will be built using the request's "

ironic/drivers/modules/oneview/common.py

@@ -44,7 +44,6 @@ opts = [
                 default=False,
                 help=_('Option to allow insecure connection with OneView')),
     cfg.StrOpt('tls_cacert_file',
-               default=None,
                help=_('Path to CA certificate')),
     cfg.IntOpt('max_polling_attempts',
                default=12,

ironic_tempest_plugin/config.py

@@ -67,7 +67,6 @@ BaremetalGroup = [
     #                 "Takes longer since Kilo as Ironic performs an extra "
     #                 "step in Node cleaning.")
     cfg.StrOpt('min_microversion',
-               default=None,
                help="Lower version of the test target microversion range. "
                     "The format is 'X.Y', where 'X' and 'Y' are int values. "
                     "Tempest selects tests based on the range between "