Backward compatible variable_product_sync()

Adds new static sync() method to WC_Product_Variable and restores
variable_product_sync() to its former, non-static glory. Also makes
the $product_id param optional on variable_product_sync() to avoid
breaking backward compatibility.
Set default $product_id on variable_product_sync()
Merge pull request #3569 from thenbrent/non_static_variable_product_sync

Backward compatible variable_product_sync()