fix(logstash): Allow managing "logstash" index

When data streams are disabled in Logstash's "elasticsearch" output
('data_stream => false'), Logstash falls back to managing "logstash-*"
indices and creates an index template with ILM enabled.
In this process, a write index for the alias "logstash" is created. This
step requires the "manage" privilege on the "logstash" alias.

Fixes #679

setup/roles/logstash_writer.json

@@ -17,6 +17,15 @@
         "manage",
         "manage_ilm"
       ]
+    },
+    {
+      "names": [
+        "logstash"
+      ],
+      "privileges": [
+        "write",
+        "manage"
+      ]
     }
   ]
 }