Allow scheduling with arbitrary user-defined resource labels. (#1236)

* Enable scheduling with custom resource labels.

* Fix.

* Minor fixes and ref counting fix.

* Linting

* Use .data() instead of .c_str().

* Fix linting.

* Fix ResourcesTest.testGPUIDs test by waiting for workers to start up.

* Sleep in test so that all tasks are submitted before any completes.

test/multi_node_test.py

@@ -205,7 +205,8 @@ class StartRayScriptTest(unittest.TestCase):
                                  "--object-manager-port", "12345",
                                  "--num-cpus", "100",
                                  "--num-gpus", "0",
-                                 "--redis-max-clients", "100"])
+                                 "--redis-max-clients", "100",
+                                 "--resources", "{\"Custom\": 1}"])
         subprocess.Popen(["ray", "stop"]).wait()
 
         # Test starting Ray with invalid arguments.