(PUP-3605) use correct usermod flag on OpenBSD for group management

this allows 'membership => inclusive' to remove groups if needed as
'-G' only appends on OpenBSD and doesn't set.

The spec test is slightly unrelated in that it ensures the creation still uses `-G`, but currently there are no tests available for `objectadd.rb` (should be added, but it's out of scope for this PR).
Does this debug statement always need to be present or was it just used for debugging the provider during development?