rbd-wnbd: use the right AdminSocket instance

rbd-wnbd: use the right AdminSocket instance

The rbd-wnbd daemon currently caches one rados context per cluster.
However, it's registering hooks against the global context
admin socket, which won't be available. For this reason,
the "rbd-wnbd stats" command no longer works.

To address this issue, we'll ensure that rbd-wnbd sets command hooks
against the right admin socket instance, leveraging the image
context.

Signed-off-by: Lucian Petrut <lpetrut@cloudbasesolutions.com>