Allow admin to do dry run of script

[ro-ket]

Add an option, such as --check, that checks to see if users or groups or uids or gids that would be created already exist. Without actually changing the onefs environment, the results of those checks are sent to stdout.

[ghost]

I started down the path of considering what to do about the users if the UID or GID already existed and it makes sense for this because I would use the same function for both.