Use CONF.import_opt() for nova.config opts

The only reason for importing nova.config now is where one of the
options defined in that file is needed. Rather than importing
nova.config using an import statement, use CONF.import_opt() so
that it is clear which option we actually require.

In future, we will move many options out of nova.config so many
of these import_opt() calls will either go away or cause a module
other than nova.config to be imported.

Change-Id: I0646efddecdf2530903afd50c1f4364cb1d5dce1

bin/nova-network

@@ -41,6 +41,7 @@ from nova import service
 from nova import utils
 
 CONF = cfg.CONF
+CONF.import_opt('network_topic', 'nova.config')
 
 if __name__ == '__main__':
     config.parse_args(sys.argv)