Implement rbd-nbd as mapping commands

[isodude]

Hi,

In later releases of Ceph there's the command rbd-nbd that presents a nbd devices instead of a kernel rbd.

In my case I wanted to go with a newer version of Ceph than shipped with the kernel, as rbd-nbd is shipped with the Ceph release, it is always up to date.

Everything can be mapped from the rbd command.

rbd map -> rbd-nbd map / rbd nbd map rbd unmap > rbd-nbd unmap / rbd nbd unmap rbd showmapped -> rbd-nbd list-mapped / rbd nbd ls

There is a lack of choosing output format with rbd-nbd though, should be ok with rbd nbd. There will also need to be control of how many nbd devices there are mapped, so that we don't hit the limit.

A process will start and serve the connection, this process needs to stay alive. One way I thought about doing it is to implement it as it's own docker instance.

Regards Josef