engine: Change AuditLogableBase.customId type to string

It makes sense to support customId be a String, so the
clients of this method shouldn't take care of converting
fields such as UUID or any other complex objects into an
Integer which guarantees narrower values range.

Change-Id: Ie8d9bcb4b6aa6046025b829669b743b672fba684
Signed-off-by: Moti Asayag <masayag@redhat.com>