Degrade gracefully if node.memory is not defined

On Windows, Ohai does not set node.memory.  If the elasticsearch cookbook is listed as a dependency on a Windows box (even if it is not used on Windows), this will cause chef to fail.

This hardcodes memory limits if node.memory is not available.

We use depend on the elasticsearch cookbook in one of our base cookbooks that applies to all systems (including Windows).  We do not actually include recipes from elasticsearch on Windows and have no intention of using the elasticsearch cookbook on Windows.  But merely including it in metadata.rb is enough to cause Windows chef runs to fail.
It's a bit scary to hardcode this limit here... Even though it defaults to the Elasticsearch's default of 1GB. What if we flip this, and simply don't set the attribute, and then check for existence eg. in `elasticsearch-env.in.sh.erb` (and other potential places where it's used)?