Re-add expose to ensure it gets run on all nodes

Previous change in a07e3301b1 meant that if there are no pods on
this node then the 'expose' never gets run.

prog/weave-kube/launch.sh

@@ -155,4 +155,7 @@ if [ ! -f /etc/cni/net.d/10-weave.conf ] ; then
 EOF
 fi
 
+# Expose the weave network so host processes can communicate with pods
+/home/weave/weave --local expose $WEAVE_EXPOSE_IP
+
 wait $WEAVE_PID

site/kube-addon.md

@@ -126,3 +126,6 @@ The list of variables you can set is:
 * IPALLOC\_INIT - set the initialization mode of the [IP Address
   Manager](/site/operational-guide/concepts.md#ip-address-manager)
   (defaults to consensus amongst the KUBE\_PEERS)
+* WEAVE\_EXPOSE\_IP - set the IP address used as a gateway from the
+  Weave network to the host network - this is useful if you are
+  configuring the addon as a static pod.