Title: health_check: Endpoint health_check_config updates not taking effect

Description:
The health_check_config configuration (i.e. active healthchecking on a different port than the application) does not get applied on endpoint config update.

The specifics of what i've tested are using file-based xDS and then updating the EDS configuration to add a health_check_config. Even though the configuration is updated successfully, the cluster healthchecker does not use this new config.

Repro steps:

I've created a (not-so-minimal) repro for this behavior which is located here: https://github.com/bplotnick/envoy_examples/tree/master/healthcheck_updates

The instructions are in the README there, but it is simply enabling healthchecking and then modifying the EDS file such that it should healthcheck against a different port which, depending on what configuration is being loaded, will either cause healthchecking to begin succeeding or failing