setup-ssh: fix updating of authorized_keys

Due to what seems like a bug (or inconsistency) in paramiko, files
opened with a+ over SFTP need a seek() in order for the user to be able
to read data from them. We implement this, and rely on the fact that we
do iterate over all lines before writing and that the file is opened in
append mode (which at least on Linux should work correctly).

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