Refactors site default merging to built in default attribute implementation

This patch removes the need for the site defaults to be merged at run time.

Instead, the compile time default attributes functionality is used to populate default values for each site.
This has the benefit of enabling other cookbooks or recipes to see the site attributes as the site recipes see them.

This work has been done as a precursor to https://github.com/inviqa/chef-config-driven-helper/issues/12 in order to split the SSL cert population in to a separate cookbook as part of that issue.