fix -G delimiter in call to useradd

The -G flag to `useradd` is currently broken when using `users.mutableUsers=true`; it currently separates groups by space, where it should use use a comma between each group.

You can see the effect by looking at my activation script before applying this fix:
https://gist.github.com/cstrahan/805050545a13a0e1cfcd#file-activate-sh-L52

I've manually tested this with both zero and more than one user. How would one go about writing an automated test for this case?