functional/047: fix md test

If we use a different WD for this test, the objects can be stored to
other disks than d2.

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
functional: add wrapper _cluster_format

This introduces a helper _cluster_format to wait for all the sheeps to
be running.  This removes sleep after 'collie cluster format' and
reduces the possibility of causing a false alarm.

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>