modules/nilrt_ip.py: Add default value for gateway
modules/nilrt_ip.py: Add default value for gateway

In case that connman cannot provide a default gateway, then this should
have a default value (0.0.0.0) to keep consistency between various ip
modules.

Signed-off-by: Alexandru Vasiu <12db13f1807ddd2f5e9e972458aecda3170fb20d@ni.com>

saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt