hc filter: treat degraded hosts as healthy in non-passthrough mode (#5980)

Updates the health check filter to treat degraded hosts as healthy for
the purpose of computing the "health" of each cluster. This captures the
idea that these hosts are still available for routing: if the upstream
cluster is fully degraded it's still available, so there shouldn't be a
need for Envoy to declare itself unhealthy.

Signed-off-by: Snow Pettersen <snowp@squareup.com>

Mirrored from https://github.com/envoyproxy/envoy @ 2e6bd24617f9c9c7085353095e196008370821c6

envoy/config/filter/http/health_check/v2/health_check.proto

@@ -32,7 +32,7 @@ message HealthCheck {
 
   // If operating in non-pass-through mode, specifies a set of upstream cluster
   // names and the minimum percentage of servers in each of those clusters that
-  // must be healthy in order for the filter to return a 200.
+  // must be healthy or degraded in order for the filter to return a 200.
   map<string, envoy.type.Percent> cluster_min_healthy_percentages = 4;
 
   // Specifies a set of health check request headers to match on. The health check filter will