Since the rsync/ssh calls are done in parallel, they can't read properly a
password or confirmation about keys from stdin. As such, it's better to enable
batch mode so that they fail right away instead of prompting and then timing
out after a long while.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>