Change healthcheck query to use the "site" label

[robertodauria]

Now that we have the "site" label on all the blackbox-exporter targets, we can avoid using label_replace.

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This change is 

[stephen-soltesz]

Reviewed 2 of 2 files at r1.
Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @evfirerob)

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healthcheck/prometheus.go, line 20 at r1 (raw file):

	// tests on the node is applied. This makes sure we never reboot a node
	// unnecessarily or lose data.
	NodeQuery = `sum_over_time(probe_success{service="ssh", module="ssh_v4_online"}[%[1]dm]) == 0

Ah, nice. This reads much more easily.

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promtest/prometheus_testing.go, line 28 at r1 (raw file):

type response struct {
	value model.Value
	err   api.Error

Huh. Strange choice on the part of the prometheus client authors, imo. I would expect an error type that could be type asserted into the underlying api.Error. This pattern is common in system packages for example. I wonder what encouraged them to use the more explicit approach.