core: align luns volume_group_id

LUNs that were previously persisted using REST might have a null volume group id
while the current value used in the engine is "", this patch fixes it and set it
to "" as well.

Change-Id: Ia3f3055ae1496a4ad5f041933fcabbd1dadee2b1
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=906360
Signed-off-by: Liron Aravot <laravot@redhat.com>