[Java] Simplify Java worker configuration (#2938)

## What do these changes do?
Previously, Java worker configuration is complicated, because it requires setting environment variables as well as command-line arguments.

This PR aims to simplify Java worker's configuration. 
1) Configuration management is now migrated to [lightbend config](https://github.com/lightbend/config), thus doesn't require setting environment variables.
2) Many unused config items are removed.
3) Provide a simple `example.conf` file, so users can get started quickly.
4) All possible options and their default values are declared and documented in `ray.default.conf` file.

This PR also simplifies and refines the following code:
1) The process of `Ray.init()`.
2) `RunManager`.
3) `WorkerContext`. 

### How to use this configuration?
1. Copy `example.conf` into your classpath and rename it to `ray.conf`.
2. Modify/add your configuration items. The all items are declared in `ray.default.conf`.
3. You can also set the items in java system prosperities.

Note: configuration is read in this priority:
System properties > `ray.conf` > `ray.default.conf`

## Related issue number
N/A

java/tutorial/README.rst

@@ -7,12 +7,12 @@ Ray Java Tutorial
 Exercises
 ---------
 
-Each file ``java/example/src/main/java/org/ray/exercise/Exercise*.java`` is a separate exercise.
-To run a exercise case, set the ``RAY_CONFIG`` env variable and run the following command in ``ray/java/`` directory.
+Each file of ``java/example/src/main/java/org/ray/exercise/Exercise*.java`` is a separate exercise.
+To run them, execute the following command under ``ray/java`` folder.
 
 .. code-block:: shell
 
-    java -Djava.library.path=../build/src/plasma/:../build/src/local_scheduler/ -classpath "tutorial/target/ray-tutorial-1.0.jar:tutorial/lib/*" org.ray.exercise.Exercise01
+    java -Dray.home=.. -classpath "tutorial/target/ray-tutorial-1.0.jar:tutorial/lib/*" org.ray.exercise.Exercise01
 
 `Exercise 1 <https://github.com/ray-project/ray/tree/master/java/tutorial/src/main/java/org/ray/exercise/Exercise01.java>`_: Define a remote function, and execute multiple remote functions in parallel.
 

java/tutorial/src/main/resources/ray.conf

@@ -0,0 +1,5 @@
+ray{
+  home: ".."
+  run-mode: CLUSTER
+  redirect-output: false
+}