Merge branch '337-fix-corednsdown-prometheus-alert' into 'master'

Resolve "Fix `CoreDnsDown` prometheus alert"

Closes #337

See merge request openappstack/openappstack!98

helmfiles/values/prometheus.yaml.gotmpl

 # Depending on which DNS solution you have installed in your cluster enable the right exporter
 coreDns:
-  enabled: false
+  enabled: true
 
 kubeDns:
-  enabled: true
+  enabled: false
 
 alertmanager:
   alertmanagerSpec: