Merge pull request #150 in ZP/z-push from bugfix/ZP-641-filestatemachine-backend-storage-file to develop

* commit '5c7a4b19':
  ZP-641 Delete file only if it exists.
  ZP-641 CleanStates() has to stop after deleting one BS file.
  ZP-641 call GetState() with the "do not clean" flag, changed CleanStates() to delete exact bs files when being calles without uuid for bs type (needs better fixing in ZP-835).