Use more sensible "default" for [oslo_messaging_rabbit] processname

Use more sensible sample default value instead of the program name to
generate the sample config file, so that users can more easily
understand the processname represents a name of the service processes.

Change-Id: Ia97b8d5b6aaa1d54d72945f3abcf4046e39c2bed

oslo_messaging/_drivers/impl_rabbit.py

@@ -257,6 +257,7 @@ rabbit_opts = [
                help='Hostname used by queue manager. Defaults to the value '
                'returned by socket.gethostname().'),
     cfg.StrOpt('processname',
+               sample_default='nova-api',
                default=os.path.basename(sys.argv[0]),
                help='Process name used by queue manager'),
     cfg.BoolOpt('rabbit_stream_fanout',