change the exposed behavior of the service resolver Subsets.OnlyPassing flag

[rboyer]

In the 1.6.x beta we exposed a field named OnlyPassing when defining a service resolver subset:

OnlyPassing (bool: false) - Specifies the behavior of the resolver's health check filtering. If this is set to false, the results will include instances with checks in the passing as well as the warning states. If this is set to true, only instances with checks in the passing state will be returned.

This was carried over verbatim from the equivalent field on Prepared Queries.

The problem is that when rendering endpoints in an envoy cluster, we want to include all endpoints regardless of health and just label them with the appropriate health status. This lets envoy calculate percentages of healthy vs overall cluster sizes and trigger failover behavior.

We already do include all of the endpoints with the following logic:

1. Service instances in a passing status in Consul become HEALTHY in Envoy.
2. Service instances in a critical status in Consul become UNHEALTHY in Envoy.
3. Service instances in a warning status in Consul become HEALTHY unless it's warning weight was set to 0.
4. Consul service instance weights affect the Envoy load balancing weight for the endpoint.

The ultimate option we actually want is to give the user the ability to modify step 3 to consider warning as always being UNHEALTHY in envoy, rather than filtering out the results.