Also dd GPT labels

wipefs may have problems with GPT labels on 4k disks. Get rid of
them. dd more of the disk as well

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1158044
Change-Id: Iced15d58939b8e7e4e86d274d1c46123500e3e6d
Signed-off-by: Ryan Barry <rbarry@redhat.com>