qa: update rbd-wnbd test, retrying image rm operations

qa: update rbd-wnbd test, retrying image rm operations

The "rbd-wnbd unmap" command is currently telling the WNBD driver
to remove the mapping without contacting the rbd-wnbd daemon
and waiting for it to perform its cleanup.

For this reason, attempting to delete the image immediately after
unmapping it can fail due to existing watchers.

As a temporary solution, we'll retry the image remove operation.
At a later time, we'll update the "rbd-wnbd unmap" command to go
through the rbd-wnbd daemon, ensuring that all the necessary
cleanup is performed before returning.

While at it, we're dropping a redundant LOG.error call so that we
won't print expected exceptions.

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