def substitute_vars(str)
      return if str.nil?
      @appliance_config.variables.keys.each do |var|
        str = str.gsub("##{var}#", @appliance_config.variables[var])
      end
      str
    end
def substitute_vars(str)
      return if str.nil?
      @appliance_config.variables.keys.each do |var|
        str = str.gsub("##{var}#", @appliance_config.variables[var])
      end
      str
    end
Substitute variables in selected string.