net-plugin: bind mount /lib/modules

Otherwise, the plugin is not able to load kernel modules
(openvswitch, vxlan, vport_vxlan) required by fastdp which
makes weave to fallback to sleeve.

CAP_SYS_MODULE is required in order to load kernel modules.

prog/net-plugin/config.json

@@ -13,7 +13,8 @@
   "linux": {
     "capabilities": [
       "CAP_SYS_ADMIN",
-      "CAP_NET_ADMIN"
+      "CAP_NET_ADMIN",
+      "CAP_SYS_MODULE"
     ]
   },
   "mounts": [
@@ -40,6 +41,12 @@
       "source": "/etc/",
       "type": "bind",
       "options": ["rbind"]
+    },
+    {
+      "destination": "/lib/modules/",
+      "source": "/lib/modules/",
+      "type": "bind",
+      "options": ["rbind"]
     }
   ],
   "env": [