e2e/prometheus: Add ServiceMonitor to TestPrometheusOnlyUpdatedOnRelevantChanges
e2e/prometheus: Add ServiceMonitor to TestPrometheusOnlyUpdatedOnRelevantChanges

- Creating and watching ServiceMonitor in
TestPrometheusOnlyUpdatedOnRelevantChanges

- Refactor isDiscoveryWorking to only log on failure

coreos/prometheus-operator,jescarri/prometheus-operator,coreos/prometheus-operator,jescarri/prometheus-operator