remus drbd: Implement remus drbd replicated disk

Implement remus-drbd-replicated-checkpointing-disk based on
generic remus devices framework.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Wen Congyang <wency@cn.fujitsu.com>
Signed-off-by: Yang Hongyang <yanghy@cn.fujitsu.com>