Bind-mount /proc in V2 plugin container.

This allows to pass ARP settings from the host, e.g.:
```
$ sudo docker-runc exec 512af90f342f16dddebc7b7245985d5eb67557e76798b4033eed2ff7adaa4eb5 cat /proc/sys/net/ipv4/neigh/weave/ucast_solicit
3
$ cat /proc/sys/net/ipv4/neigh/weave/ucast_solicit
3
$ cat /proc/sys/net/ipv4/neigh/weave/ucast_solicit
1
$ sudo docker-runc exec 512af90f342f16dddebc7b7245985d5eb67557e76798b4033eed2ff7adaa4eb5 cat /proc/sys/net/ipv4/neigh/weave/ucast_solicit
1
```

prog/v2-plugin/config.json

@@ -16,7 +16,13 @@
       "CAP_NET_ADMIN"
     ]
   },
-  "mounts": [],
+  "mounts": [
+    {
+      "destination": "/procmount",
+      "type": "proc",
+      "source": "proc"
+    }
+  ],
   "network": {
     "type": "host"
   },