HAProxy verify certificate on health check

[nsg]

Add verify required for the backend servers to verify certificates on health checks.
This will result in a failed healthcheck when the wrong domain is served (for example due to a stale DNS lookup), which will trigger HAProxy to perform a new DNS lookup.

(imported from internal issue)

[nsg]

I and @splushii had a discussion, we need to be able to turn the feature on/off. Because we do not play the legacy game I personalty vote to keep this enabled by default. Maybe this needs to be both a global and per-rule toggle.