add delete option to TemporaryDirectory

if false, then the created path will not be removed
fix bug where failing lvm commands were not detected

- regression when deprecating use of backported check_output
  from subprocess.  The capture_stdout implementation from
  the LVM API was not checking the returncode properly. This
  now raises an LVMCommandError if a command exits with
  non-zero status.
implement skip-unmount option in LVM
implement LVMSnapshot.release()
resolve symlinks in BackupController.release()
add release command for holland cli