OC-10832 - AIX - group provider implementation

Implements missing functionality in AIX, mainly add/remove members from a group.

This PR depends on PR https://github.com/opscode/chef/pull/1176


we have this one aix 6.1 system which strangely does not have -f option support, and i feel we don't really need to use this in tests as -r does the necessary clean up of test users.