Add debug string to raylet (#3317)

* initial debug string

* format

* wip debug string

* fix compile

* fix

* update

* finished

* to file

* logs dir

* use temp root

* fix

* override

python/ray/services.py

@@ -955,6 +955,7 @@ def start_raylet(redis_address,
         start_worker_command,
         "",  # Worker command for Java, not needed for Python.
         redis_password or "",
+        get_temp_root(),
     ]
 
     if use_valgrind: