librbd: allow to open an image without opening parent image

librbd: allow to open an image without opening parent image

Fixes: http://tracker.ceph.com/issues/18325
Signed-off-by: Ricardo Dias <rdias@suse.com>
(cherry picked from commit 61af1c25015de087a2423811548d975dd7d430b4)

Conflicts:
  src/librbd/ImageState.cc - added missing arg to RefreshRequest::create
  src/librbd/exclusive_lock/PostAcquireRequest.cc -
                                        deleted, does not exist in jewel
  src/librbd/image/OpenRequest.cc - added missing arg to
                                    RefreshRequest::create
  src/librbd/internal.cc - added missing arg to ImageState::open
  src/librbd/librbd.cc - added missing arg to ImageState::open
  src/test/librbd/exclusive_lock/test_mock_PostAcquireRequest.cc -
                                        deleted, does not exist in jewel
  src/test/rbd_mirror/image_replayer/test_mock_CreateImageRequest.cc -
                                        added missing arg to ImageState::open
  src/test/rbd_mirror/test_PoolWatcher.cc - added missing arg to
                                            ImageState::open