Refactor operating system recipes into default.

The differences between operating systems are mainly adding the
repository mirrors (yum, apt) and generally do not require specific
software differences. If those differences do exist they should be done
explicitly in the configuration recipes for rabbitmq, etc.
Move chef-client recipe setup into worknode recipe.

A headnode is merely a worknode with additional software. Therefore it
makes sense for some software, mostly chef-client, to be only installed
as a base and not on the bootstrap node.
Fix the DNS address for Google's backup.
Add ntp cookbook as a dependency.
Always use latest version of Berkshelf.