Fix OSX Group provider to be properly idempotent

Currently the OSX Group provider use the 'Etc' module to determine
what exists, and the 'dscl' command to change things. Etc will look
to /etc/group by default and fallback to 'dscl', which causes idempotency
incorrectness. This change the Group provider to simply look at 'dscl' always.
Merge pull request #1746 from jaymzh/macosx_groups_10

Fix OSX Group provider to be properly idempotent