Log filtering "no virtual host available"

[mmatczuk]

https://github.com/koding/tunnel/blob/master/server.go#L161

if !strings.Contains(err.Error(), "no virtual host available") { // this one is outputted too much, unnecessarily
    s.log.Error("remote %s (%s): %s", r.RemoteAddr, r.RequestURI, err)
}

Such filtering could be moved to logging package for grater flexibility.

Also logging could have a filter that would print this log every 100 or so occurrences so that you can see when system is flooded with those logs.