Issue randomly present on ZyXEL GS1900-8, where it seems, that it takes
some time to get SSH ready:

 $ ssh root@$TARGET_LAN_IP "ubus call system board" | tee system-$LABGRID_TARGET.json
 ssh: connect to host 192.168.1.1 port 22: Connection refused

So lets try to fix it by checking for SSH connection several times.

Signed-off-by: Petr Štetiar <ynezz@true.cz>