of12: fix InstructionWriteMetadata json decoder

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
sw test tool: FIX: return the transaction id when flow entries were deleted

OfTester._test_initialize() expects the transaction id as a return value of TargetSw.del_test_flow() for checking the error.
but this method at current version does not return the transaction id.

this patch gets del_test_flow() to return the transaction id.

Signed-off-by: Yuichi Ito <ito.yuichi0@gmail.com>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>