Move output in task/s3readwrite

The s3readwrite.py task formerly wrote too much output while excuting.
It now saves the data on the local machine in either the archive
directory or in /tmp if no archive directory is specified.
The new file contains a client name and timestamp in its name.
Once all processing has completed, that file is saved locally.

Fixes: 9117
Signed-off-by: Warren Usui <warren.usui@inktank.com>
Merge pull request #212 from ceph/wip-9117-wusui

Move output in task/s3readwrite