test/rgw: address potential race condition in reshard testing

test/rgw: address potential race condition in reshard testing

The testing code does a `reshard add` followed by a `reshard list`. On
the narrow chance that the reshard was completed between those two
commands, we need to be able to handle the case where the reshard list
is empty. In that case we'll do a `bucket stats` to verify the reshard
completed successfully.

Signed-off-by: J. Eric Ivancich <ivancich@redhat.com>