dump internal state variables to file if debug is enabled

enabling debug mode also dumps the current value of all internal
state variables to "/tmp/state_<id>.stage#", where <id> is the
current epoch

  setup-storage2 -f /var/lib/fai/config/disk_config/01_single-disk -d -X

may provide

  state_1408642765.stage1
  state_1408642765.stage2
  state_1408642765.stage3
  state_1408642765.stage4
  state_1408642765.stage5

This output can be used to 'replay' the original execution in a testcase.