Add --tag to build options

[Demonsthere]

Hey, would it be a problem to include the possibility to set image name for build parameter in the .kitchen.yml file?

      def docker_build_command
        cmd = String.new('build')
        cmd << ' --no-cache' if config[:no_cache]
        cmd << "--tag #{config[:image_name]}" if config[:image_name]
        if config[:build_context]
          cmd << ' .'
        else
          cmd << ' -'
        end
      end