Update variable name to standard

It was agreed to name role variables as `r_ROLE_NAME_VARIABLE_NAME`. Giving it a try.

roles/openshift_health_checker/openshift_checks/etcd_volume.py

@@ -28,7 +28,7 @@ class EtcdVolume(OpenShiftCheck):
 
         threshold = get_var(
             task_vars,
-            "etcd_device_usage_threshold_percent",
+            "r_openshift_health_checker_etcd_device_usage_threshold_percent",
             default=self.default_threshold_percent
         )