task/workunit: add sudo to dir delete

This failure to delete used to be hidden in
a catch-all exception handler that was
removed in 396dc073.

This was causing failures in kcephfs suite
like:
Command failed on plana96 with status 1: 'rm -rf -- /home/ubuntu/cephtest/mnt.0/client.0'

Signed-off-by: John Spray <john.spray@redhat.com>