Wipe disks added through "gnt-instance modify"

In issue 353 Sascha Lucas reported that disks are not wiped when added
through “gnt-instance modify”. This patch adds this functionality and
updates the docstring for “_WipeDisks”.

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