for health check, use prom healthy endpoint by default

[jmazzitelli]

fixes: #6966

[jmazzitelli]

to test, just deploy everything normally and get into a good state. Then scale down the prometheus deployment to 0 pods:

kubectl scale --replicas=0 deployment -n istio-system prometheus

and then look at the Kiali UI and wait for it to refresh. You should get the "unreachable" masthead message (along with some other bad errors because prometheus is down, but ignore those, that's not part of this issue).

Then scale up the prom deployment back to 1:

kubectl scale --replicas=1 deployment -n istio-system prometheus

and the Kiali UI should go back to normal - you should no longer see the "unreachable" masthead message.

[jshaughn]

Ran the test instructions successfully. LG