test: change the branch of openvswitch to 'of12'

Change the branch of openvswitch used by integration tests to 'of12'

Signed-off-by: HIYAMA Manabu <hiyama.manabu@po.ntts.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>