Moved computation of `dowload_url` to the recipe

This prevents a number of errors and surprises when setting the Elasticsearch version.

If the user overrode the node.elasticsearch[:version] property, the download directory and symlinks would be
created with the correctly overridden version number. However, the download URL would be computed with
the default version number, and thus the incorrect, default version of elasticsearch would be installed.

Since there's no need to compute the download filename and URL until node convergence, this patch moves that computation to the default.rb recipe. However, the node.elasticsearch[:filename] and node.elasticsearch[:download_url] attributes, if present, are respected and given preference upon node convergence, so that users still have the capability to override these attributes and thus configure downloads from a custom repository.

Related: #100, #180, #178, #240, #179, #245, #227, #245

Closes #242