Handle interfaces that only have link-local addrs

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

website/source/docs/agent/configuration/client.html.md

@@ -92,7 +92,7 @@ client {
  "client", like `"/opt/nomad/client"`. This must be an absolute path.
 
 - `gc_interval` `(string: "1m")` - Specifies the interval at which Nomad
-  attempts to garbage collect terminal allocation directories. 
+  attempts to garbage collect terminal allocation directories.
 
 - `gc_disk_usage_threshold` `(float: 80)` - Specifies the disk usage percent which
   Nomad tries to maintain by garbage collecting terminal allocations.
@@ -268,6 +268,19 @@ see the [drivers documentation](/docs/drivers/index.html).
     }
     ```
 
+- `"fingerprint.network.disallow_link_local"` `(string: "false")` - Specifies
+  whether the network fingerprinter should ignore link-local addresses in the
+  case that no globally routable address is found. The fingerprinter will always
+  prefer globally routable addresses.
+
+    ```hcl
+    client {
+      options = {
+        "fingerprint.network.disallow_link_local" = "true"
+      }
+    }
+    ```
+
 ### `reserved` Parameters
 
 - `cpu` `(int: 0)` - Specifies the amount of CPU to reserve, in MHz.