Merge branch 'devel-2.3'

* devel-2.3:
  Update ganeti-os-interface documentation
  Makefile: Check for empty files and dirs on distcheck
  Revert commit e7e23e73, work around Automake bug

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>