tunnel-push-pop.at: Make test more deterministic.

Using the '-march=native' broke this test in a i7 laptop, most likely
due to a different hash implementation, which resulted in different
datapath port number assignment accross different compilations.

Break up the ports setup so that the datapath port number assignment
is more deterministic.

Signed-off-by: Jarno Rajahalme <jrajahalme@nicira.com>
Acked-by: Pravin B Shelar <pshelar@nicira.com>