Add ML challenge

With this commit we add a new ML-related challenge to our benchmarks.

Closes elastic#39
Relates elastic#84

Author: danielmitterdorfer

night_rally/resources/tracks.json

@@ -223,6 +223,15 @@
         "challenge": "append-no-conflicts",
         "car": "4gheap"
       },
+      {
+        "name": "nyc_taxis-ml-4g-1node",
+        "label": "ml-4g",
+        "#COMMENT": "Don't generate any standard charts but we'll create ML-specific charts",
+        "charts": [],
+        "challenge": "append-ml",
+        "car": "4gheap",
+        "x-pack": ["ml"]
+      },
       {
         "name": "nyc_taxis-append-4g-x-pack-security-1node",
         "label": "add-4g-security",