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

neutron/agent/metadata/config.py

@@ -36,7 +36,6 @@ SHARED_OPTS = [
 
 DRIVER_OPTS = [
     cfg.BoolOpt('metadata_proxy_watch_log',
-                default=None,
                 help=_("Enable/Disable log watch by metadata proxy. It "
                        "should be disabled when metadata_proxy_user/group "
                        "is not allowed to read/write its log file and "

neutron/agent/metadata/namespace_proxy.py

@@ -158,11 +158,9 @@ def main():
                    help=_('Location of Metadata Proxy UNIX domain '
                           'socket')),
         cfg.StrOpt('metadata_proxy_user',
-                   default=None,
                    help=_("User (uid or name) running metadata proxy after "
                           "its initialization")),
         cfg.StrOpt('metadata_proxy_group',
-                   default=None,
                    help=_("Group (gid or name) running metadata proxy after "
                           "its initialization")),
         cfg.BoolOpt('metadata_proxy_watch_log',

neutron/common/config.py

@@ -72,10 +72,10 @@ core_opts = [
                help=_("Maximum number of fixed ips per port. This option "
                       "is deprecated and will be removed in the N "
                       "release.")),
-    cfg.StrOpt('default_ipv4_subnet_pool', default=None,
+    cfg.StrOpt('default_ipv4_subnet_pool',
                help=_("Default IPv4 subnet-pool to be used for automatic "
                       "subnet CIDR allocation")),
-    cfg.StrOpt('default_ipv6_subnet_pool', default=None,
+    cfg.StrOpt('default_ipv6_subnet_pool',
                help=_("Default IPv6 subnet-pool to be used for automatic "
                       "subnet CIDR allocation")),
     cfg.IntOpt('dhcp_lease_duration', default=86400,
@@ -113,7 +113,7 @@ core_opts = [
                 help=_('If True, effort is made to advertise MTU settings '
                        'to VMs via network methods (DHCP and RA MTU options) '
                        'when the network\'s preferred MTU is known.')),
-    cfg.StrOpt('ipam_driver', default=None,
+    cfg.StrOpt('ipam_driver',
                help=_('IPAM driver to use.')),
     cfg.BoolOpt('vlan_transparent', default=False,
                 help=_('If True, then allow plugins that support it to '

neutron/tests/tempest/config.py

@@ -73,7 +73,6 @@ IdentityGroup = [
                 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',