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.
I'd really liked something like `<%= node.elasticsearch[:allocated_memory] ? "ES_HEAP_SIZE=#{node.elasticsearch[:allocated_memory]}" : ""`, i.e. not hardcoding a specific numerical value in the cookbook. (Writing the code blindly, not testing it.)