core: WAD should be Ignored on File Domain Disks

Since the file system is responsible for handling block allocation,
there is no need for wiping disks on file domains.

Change-Id: I0df2c07282994557e149db922cc1d3a166c5aa8f
Bug-Url: https://bugzilla.redhat.com/1097820
Signed-off-by: Idan Shaby <ishaby@redhat.com>