Use some other constant

Thanks for this great drush feature!
I'm creating a lot of development site on localhost, and I was angry, always type same. In this patch i create some database constant:
+define ('DENSITE_DB_DRIVER', NULL);
+define ('DENSITE_DB_HOST', NULL);
+define ('DENSITE_DB_USERNAME', NULL);
+define ('DENSITE_DB_PASSWORD', NULL);

If used, these are printed in cli.