cfgupgrade: Add confirmation message

A message will be given instead of just dropping the user back to the
prompt in case of a successful upgrade.

[…]
documentation formats). Continue with upgrading configuration?
y/[n]/?: y
Configuration successfully upgraded for version 2.5.0~beta1.
root@master:~#

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>

tools/cfgupgrade

@@ -248,6 +248,9 @@ def main():
     del cfg
     logging.info("File loaded successfully")
 
+  cli.ToStderr("Configuration successfully upgraded for version %s.",
+               constants.RELEASE_VERSION)
+
 
 if __name__ == "__main__":
   main()