create_user and groups option

If a user account does not exist yet on the (Linux) remote, and create_user command is being run with a groups option, then the first group will be treated as primary group for the user, therefore it will get passed as --gid primary_group to useradd, stg like this:

useradd --gid primary_group --groups group2,group3 username

In this case useradd fails with a useradd: group 'primary_group' does not exist error. I think we should detect this case, create the user without any group settings, and apply the group membership settings once the account has been created, just before returning.

This way useradd can do its job (optionally creating the user's default primary group based on the remote system's default settings), and then ensure the requested group memberships.

For bonus points, there could be a force_groups => TRUE option or similar, which forcibly creates all the missing groups, before useradd is called.

As far as I can tell, the above does not affect the case when the account is already there (e.g. create_user acts in usermod mode).