utils.CreateBackup: Use human-readable instead of seconds since Epoch

Seconds since the Epoch are not easily readable by a human. Using a
formatted timestamp makes it easier (e.g.
“….backup-2010-03-12_14_02_43.…”). This patch also makes OS logfiles use
this formatted timestamp.

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