Title: Envoy should support client side protocol negotiation via alpn

Description:
At present if the an upstream cluster has http2_protocolOptions, it will always send h2 traffic upstream. When used with ALPN = {"h2", "http/1.1"} the expectation is that it will be negotiated down to http/1.1if the server only supporthttp/1.1` I don't think this is happening.

We need this in Istio so that we can set "ALPN" options on all outbound tls connections and upgrade.

We also need to have http/1.1 --> h2c, there is a separate issue #1502 filed for this.