s3readwrite.py: enable overrides

Enable s3readwrite task to have the branch to
download specified and for overrides to be
incorporated into the config at run-time.
Code based on the s3tests.py task.

Signed-off-by: Joe Buck <jbbuck@gmail.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>
Merge remote-tracking branch 'gh/next'