[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_exec_interceptor.go

@@ -29,7 +29,7 @@ func (i *createExecInterceptor) 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("Exec in container %s with WEAVE_CIDR \"%s\"", container.ID, strings.Join(cidrs, " "))
 		cmd := append(weaveWaitEntrypoint, "-s")
 		options.Cmd = append(cmd, options.Cmd...)