Allow run fitnesse on a slave with a different OS than master's one

- Allow run fitnesse on a slave with a different OS than master's one
  * never use java.io.File API
  * always transform relative to absolute path
  * add logs
- Use overridden JDK on slave instead of master's one
  * take declared JDK
  * if none declare, use $java_home
  * otherwise, take launch java command in $path

- Fix NPE when JDK is not set in jenkins global configuration