Handle ip route showing mask-less IP addresses

Sometimes `ip route` will show mask-less IPs, so net.ParseCIDR will fail. If it does we check if we can net.ParseIP, and fail only if we can't.
Fixes #1214
Fixes #362
testing, issue #1331: Add registry functional test to docker-ci
Make sure the routes IP are taken into consideration + add unit test for network overlap detection
Merge pull request #1374 from dotcloud/steeve-patch-1

- Runtime: Handle ip route showing mask-less IP addresses