waiting for network probing up may fail before default timeout

I observed wait_network_up could fail on some of my board for the network probe up with a latency, it will fail before timeout.

So a previous patch of mine can't work, I suggest to use time.sleep(timeout) to make it more reliable.

The logs from my board, it only detect 4 times and network reports failure, but the timeout is far away reached:

 * Documentation: http://www.linaro.org
                                                                         [ OK ]
root@master:~# Waiting for network to come up
LC_ALL=C ping -W4 -c1 192.168.12.8
LC_ALL=C ping -W4 -c1 192.168.12.8
connect: Network is unreachable
root@master:~# LC_ALL=C ping -W4 -c1 192.168.12.8
LC_ALL=C ping -W4 -c1 192.168.12.8
connect: Network is unreachable
root@master:~# LC_ALL=C ping -W4 -c1 192.168.12.8

root@master:~# Lava failed at action deploy_linaro_image with error: Network can't probe up when deployment