[diagnostics] show health-check response
arvados-client sudo diagnostics should perform a cluster-wide health check. This should work even if the "health aggregator" service is not running.
(Note we can't do this in the non-sudo case because arvados-client won't have the management token needed to query health-check endpoints. In general,
arvados-client diagnostics doesn't even run on the cluster's internal network, so it can't necessarily connect to all internal services.)
Add text to install guide about how to use the diagnostics and health check to establish confidence that a cluster is correctly installed & working.
Do this assuming correct information from the config file. We suspect some clusters may not have usable InternalURL values, in these cases we will fix the configuration and/or the installer.