Using ptp plugin with non default routes, we get the following error
when cri-o call CheckNetworkList():
```
Expected Route {Dst:{IP:198.18.128.0 Mask:ffff8000} GW:<nil>} not found in routing table
```
Using cniVersion 0.3.1 to bypass the check, we can see that the
route is added with a gateway
```
$ ip r
198.18.0.0/17 via 198.18.0.1 dev eth0 src 198.18.3.102
198.18.0.1 dev eth0 scope link src 198.18.3.102
198.18.128.0/17 via 198.18.0.1 dev eth0
```
If GW is nil only check if we have a route with a DST that matches, and
ignore the GW.
Fixes #886
Signed-off-by: Etienne Champetier <e.champetier@ateme.com>
c347755f87