Move output in task/s3readwrite

The s3readwrite.py task formerly wrote too much output while excuting.
It now saves the data to a StringIO value that gets written upon
completion and only gets displayed if verbose is set to True in the yaml
file.

Fixes: 9117
Signed-off-by: Warren Usui <warren.usui@inktank.com>
This returns to the behavior of streaming the output over the wire to a local file...