Automatically add relevant directories to Python paths of workers (#380)

* Make ray.init set python paths of workers.

* Decouple starting cluster from copying user source code

* also add current directory to path

* Add comments about deallocation.

* Add test for new code path.

test/runtest.py

@@ -351,6 +351,23 @@ class APITest(unittest.TestCase):
 
     ray.worker.cleanup()
 
+  def testRunningFunctionOnAllWorkers(self):
+    ray.init(start_ray_local=True, num_workers=1)
+
+    def f():
+      sys.path.append("fake_directory")
+    ray.worker.global_worker.run_function_on_all_workers(f)
+    @ray.remote([], [list])
+    def get_path():
+      return sys.path
+    self.assertEqual("fake_directory", ray.get(get_path.remote())[-1])
+    def f():
+      sys.path.pop(-1)
+    ray.worker.global_worker.run_function_on_all_workers(f)
+    self.assertTrue("fake_directory" not in ray.get(get_path.remote()))
+
+    ray.worker.cleanup()
+
 def check_get_deallocated(data):
   x = ray.put(data)
   ray.get(x)