move default attributes that reference string interpolated attributes into recipe
update passenger recipe to accept any attributes.  Anything attribute added to node["nginx"]["passenger"] except root and gem_binary will now be written out to passenger.conf with the string "passenger_" prepended.
e.g. node["nginx"]["passenger"]["buffers"] = "8 16k" would result in a line in passenger.conf like this
passenger_buffers 8 16k;
where it previously was absent from the list of configurable options.
Configuration items with nil values are not written to the configuration
file at all, so default values that are overridden to nil can be
removed.