tests: Minor cleanup to librbd test

The tests can now be repeated to increase the chance of hitting
edge condition failures.  Also added additional logic to immediately
fail IO tests when an error is encountered.

Signed-off-by: Jason Dillaman <dillaman@redhat.com>
Merge pull request #3141 from ceph/wip-librbd-test-cleanup

tests: Minor cleanup to librbd test

Reviewed-by: Josh Durgin <jdurgin@redhat.com>