Added tests for /_cluster/allocation/explain. (#523)

Signed-off-by: dblock <[email protected]>

tests/default/cluster/allocation/explain.yaml

@@ -0,0 +1,31 @@
+$schema: ../../../../json_schemas/test_story.schema.yaml
+
+description: Test cluster allocation explain requests.
+epilogues:
+  - path: /games
+    method: DELETE
+    status: [200, 404]
+prologues:
+  - path: /{index}
+    method: PUT
+    parameters:
+      index: games
+chapters:
+  - synopsis: Finds an unassigned shard and explains why it can't be allocated to a node (GET).
+    path: /_cluster/allocation/explain
+    method: GET
+    response:
+      status: 200
+  - synopsis: Finds an unassigned shard and explains why it can't be allocated to a node (POST).
+    path: /_cluster/allocation/explain
+    method: POST
+    parameters:
+      include_yes_decisions: true
+      include_disk_info: true
+    request:
+      payload:
+        index: games
+        shard: 0
+        primary: true
+    response:
+      status: 200