runtime: correctly detect IPv4 forwarding

When memory cgroup is absent, there was not attempt to detect if IPv4
forwarding was enabled and therefore, docker was printing a warning
for each command spawning a new container. The test for IPv4
forwarding was guarded by the test for memory cgroup.
Merge pull request #1497 from vincentbernat/fix/ipv4-forwarding-detection

runtime: correctly detect IPv4 forwarding