core: Configurable Default for Wipe After Delete per Storage Domain

This patch adds the ability to configure a default value for wipe after
delete (WAD) per storage domain via the REST API. This way, each new
disk will get the default value for WAD from its storage domain.
Note that SANWipeAfterDelete will no longer be the WAD default value for
disks, but for storage domains.

Change-Id: I4e0d5a1c9158be1284ffe05d1bf1397c9113f6c0
Bug-Url: https://bugzilla.redhat.com/1073448
Signed-off-by: Idan Shaby <ishaby@redhat.com>
Signed-off-by: Tal Nisan <tnisan@redhat.com>