Update secrets aws_credentials data bag item
Add S3 configuration sharing for Exhibitor

Allows for Exhibitor to auto-manage the Zookeeper ensemble, using a
single, shared config file stored on S3.

See
https://github.com/Netflix/exhibitor/wiki/Shared-Configuration
for details

The `node[:exhibitor][:opts]` attributes specified are picked up by
https://github.com/SimpleFinance/chef-zookeeper/blob/49b9d05/templates/default/exhibitor.upstart.conf.erb#L12