change default group mapping for SLES to gpasswd

gpasswd is the new default starting with SLES12, the groupmod command
does not support the -A/-R commands for adding and removing users
respectively
Merge branch 'suse-group' of github.com:mapleoin/chef

Conflicts:
	spec/unit/platform_spec.rb
Merge pull request #2211 from opscode/sersut/rebase-chef-2187

Rebase CHEF-2187: change default group mapping for SLES to gpasswd