Disallow public posting from groups completely

Issue Summary

This issue comes from the perspective of, say, a site admin who would like to disable public posting from groups completely, including from users who have the ability to create a group-type node.

The 'Do not allow public posts in this group?' option appears whenever creating/editing a group-type node, and there does not appear to be any way to suppress it, and to set groups to disallow public posting by default, upon creation. I imagine this use case would be quite common for anyone trying to set up more isolated group spaces.

It shouldn't be too hard to have a setting on the OG User Roles configuration page -- something like 'Disallow Public Posting From Groups Completely'. Checking this off will suppress the option from the group node form. Also, doing so would ensure that upon group node creation, the 'og_user_roles_assign_nopublic_N' variable is set to a true value. Furthermore, it should probably set the values for all pre-existing og_user_roles_assign_nopublic_N variables to true.