prometheus: add blackbox exporter

roles/prometheus/meta/argument_specs.yml

@@ -110,6 +110,13 @@ argument_specs:
                 type: str
                 required: true
 
+      prometheus_ping_hosts:
+        description: List of hosts to gather ping metrics for using prometheus blackbox exporter
+        type: list
+        required: false
+        default: []
+        elements: str
+
       # All options after this will be passed directly to the container role
       docker_service_suffix:
         description: "Passed to container role"