Implements app_vhost definition

This PR implements a unified app_vhost implementation that is used for both apache-sites and nginx-sites helpers.

It includes the work from PR #14 to refactor the default site merging to occur at compile time and thus makes all attributes available to external cookbooks and recipes.

This PR aligns the two recipes, fixes issue #12 and enables the SSL cert population to be split to a separate recipe (I've de-duplicated here but not enabled independent usage of it).