def read_yaml_key(source, key)
      return unless key.is_a?(String)
      paths = key.split('.')
      source = source[paths.shift] until paths.empty? || source.nil?
      source
    end
def read_yaml_key(source, key)
      return unless key.is_a?(String)
      paths = key.split('.')
      source = source[paths.shift] until paths.empty? || source.nil?
      source
    end
Split a dot-separated key and locate the value from a hash loaded by YAML.
   eg. `aws.bucket` looks for `source['aws']['bucket']`.