docs(chef): update for the new manage.opscode.com

Deis docs' current reference to https://manage.opscode.com/groups/admins/edit for hosted Chef is now invalid since their web UI was updated. This corrects references to that and to preview.opscode.com, and makes clear that the client is named "deis-controller-ec2" on EC2.
Merge pull request #682 from opdemand/update-chef-admins

docs(chef): update for the new manage.opscode.com