fix(make_chroot): Fix support for new users with existing groups.

When a user creates a chroot and as a common primary group such as
'users' the groupadd command fails. Instead treat this the same as users
and only fail if the group exists but has a different (such as the
'users' group not using GID 100). Hopefully this works better.
Merge pull request #110 from marineam/group

fix(make_chroot): Fix support for new users with existing groups.