core: Remove storage format validation when adding unattached data domain

Remove storage format validation when adding an unattached data domain with
a host that doesn't have a data center

Change-Id: I4feea2d476e89e89d2b1ad572ce64904063a0914
Bug-Url: https://bugzilla.redhat.com/1141784
Signed-off-by: Tal Nisan <tnisan@redhat.com>