[proxy] Add support for WEAVE_CIDR=none

Tells the proxy not to do anything with this container. We set it when
running weaveexec/weave/etc containers, so that the weave script works
through the proxy.

proxy/create_container_interceptor.go

@@ -33,7 +33,7 @@ func (i *createContainerInterceptor) InterceptRequest(r *http.Request) error {
 		return err
 	}
 
-	if cidrs, ok := weaveCIDRsFromConfig(container.Config); ok || !i.proxy.NoDefaultIPAM {
+	if cidrs, ok := weaveCIDRsFromConfig(container.Config, i.proxy.NoDefaultIPAM); ok {
 		Info.Printf("Creating container with WEAVE_CIDR \"%s\"", strings.Join(cidrs, " "))
 		if container.HostConfig == nil {
 			container.HostConfig = &docker.HostConfig{}