Use MESOS_DIRECTORY in MesosExecutor process (not user.dir property)

We shouldn't rely on working dir (user.dir) of MesosExecutor process, for
setting  the mapred.local.dir as it can point to a directory which is not
in a mesos sandbox, nor strictly local in the first place.

src/main/java/org/apache/hadoop/mapred/MesosExecutor.java

@@ -51,7 +51,7 @@ private JobConf configure(final TaskInfo task) {
 
     // Set the mapred.local directory inside the executor sandbox, so that
     // different TaskTrackers on the same host do not step on each other.
-    conf.set("mapred.local.dir", System.getProperty("user.dir") + "/mapred");
+    conf.set("mapred.local.dir", System.getenv("MESOS_DIRECTORY") + "/mapred");
 
     return conf;
   }