At present If Mixer (istio-policy) is not available, then we get
[2018-08-14T09:26:52.187Z] "GET /apis HTTP/1.1" 503 - 0 69 4 - "10.132.0.3" "FxVersion" "bae0a141-3e55-9240-9074-8429433ffc32" "mysvc" "-"

Both UPSTREAM_HOST and RESPONSE_FLAGS are empty.

Mixer client should record the reason in RESPONSE_FLAGS

• At present use UNAUTHORIZED_EXTERNAL_SERVICE response flag
• Add UNAVAILABLE_EXTERNAL_SERVICE or equivalent flag to envoy and use it so that access logs distinguish between the two.