[TEST] Dynamically extract node ID for test

rest-api-spec/test/cluster.reroute/11_explain.yaml

@@ -26,6 +26,12 @@ setup:
 ---
 "Explain API for non-existant node & shard":
 
+  - do:
+      cluster.state:
+        metric: [ master_node ]
+
+  - set: {master_node: node_id}
+
   - do:
       cluster.reroute:
         explain: true
@@ -35,15 +41,16 @@ setup:
             - cancel:
                 index: test_index
                 shard: 9
-                node: node_0
+                node: $node_id
 
   - match: {explanations.0.command: cancel}
   - match:
       explanations.0.parameters:
         index: test_index
         shard: 9
-        node: node_0
+        node: $node_id
         allow_primary: false
   - match: {explanations.0.decisions.0.decider: cancel_allocation_command}
   - match: {explanations.0.decisions.0.decision: "NO"}
   - is_true: explanations.0.decisions.0.explanation
+