Allow override normal attributes passed by command line to chef-solo. Put to libraries/ folder of your cookbook.

override_attrs.rb

#
# Load attributes from local file
# overriding/merging with command line attributes
#
require 'chef/run_context'

class Chef::RunContext

  alias_method :original_load_attributes, :load_attributes

  def load_attributes
    filename = '/etc/chef/attrs.json'
    
    begin
      json_file = open(filename)
      json = Chef::JSONCompat.from_json(json_file.read)
      json_file.close unless json_file.closed?
      Chef::Log.debug("Node #{@node.name} loading attributes from json #{filename}")
      @node.consume_attributes(json)
    rescue Exception => error
      Chef::Log.debug("Skipping #{filename} due to error: " + error.message)
    end
    original_load_attributes
  end
end

Chef 10.x only for now.
(For Chef 11.x need to override Chef::RunContext::CookbookCompiler.compile_attributes instead)