- Fixed __hasBootLoaderTools() runtime check to check for
  the bootloader tool if 'grub' is selected
- Fixed tests for KIWIRuntimeChecker code
  * for each test call the corresponding runtime checker function
    and not the container method which always calls all of them
  * added variable KIWI_NO_FS which allows to switch off runtime
    checks which accesses the filesystem. They can only be safely
    called if the filesystem environment is controlled too