Fixed mismatch on ipvX gateway

Fixes a mismatch for configuring/retrieving gateway, two different keys were needed, ```lxc.network.index.ipv4.gateway``` and ```lxc.network.index.ipv4_gateway```, respectively. Applying the patch, only the first is used for both operations.

Also added some tests for it (copied the existing ones for ipv4 and modified).

Let me know if there's anything else that should be done.

Thanks.