[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/test/src/main/java/org/ray/api/test/RayConfigTest.java

@@ -0,0 +1,20 @@
+package org.ray.api.test;
+
+import org.junit.Assert;
+import org.junit.Test;
+import org.ray.runtime.config.RayConfig;
+import org.ray.runtime.config.RunMode;
+import org.ray.runtime.config.WorkerMode;
+
+public class RayConfigTest {
+
+  @Test
+  public void testCreateRayConfig() {
+    System.setProperty("ray.home", "/path/to/ray");
+    RayConfig rayConfig = RayConfig.create();
+
+    Assert.assertEquals("/path/to/ray", rayConfig.rayHome);
+    Assert.assertEquals(WorkerMode.DRIVER, rayConfig.workerMode);
+    Assert.assertEquals(RunMode.CLUSTER, rayConfig.runMode);
+  }
+}

java/test/src/main/java/org/ray/api/test/TestListener.java

@@ -9,6 +9,8 @@ public class TestListener extends RunListener {
 
   @Override
   public void testRunStarted(Description description) {
+    System.setProperty("ray.home", "../..");
+    System.setProperty("ray.resources", "CPU:4");
     Ray.init();
   }