Fix deprecated_name in the help message in config.py

1. vnc.vnc_enabled in nova.conf is deprecated, and vnc.enabled
   is used instead, so help message of vnc_console should be fixed
   accordingly.
2. The format "nova.conf: DEFAULT.enable_instance_password" is the
   clearest one to mention the config item in nova.conf, so change
   all the occurrences to this format.

Change-Id: Iec8c6aeafd04d6449f7c5250e9953e80eb8a9151
1 file changed