This PR changes the fingerprint handling of network interfaces that only
contain link local addresses. The new behavior is to prefer globally
routable addresses and if none are detected, to fall back to link local
addresses if the operator hasn't disallowed it. This gives us pre 0.6
behavior for interfaces with only link local addresses but 0.6+ behavior
for IPv6 interfaces that will always have a link-local address.
Fixes https://github.com/hashicorp/nomad/issues/3005
/cc diptanuc
fd7d614ae4