[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/api/pom.xml

@@ -14,4 +14,11 @@
   <description>java api for ray</description>
 
   <packaging>jar</packaging>
+
+  <dependencies>
+    <dependency>
+      <groupId>org.slf4j</groupId>
+      <artifactId>slf4j-log4j12</artifactId>
+    </dependency>
+  </dependencies>
 </project>

java/api/src/main/java/org/ray/api/Ray.java

@@ -2,7 +2,6 @@ package org.ray.api;
 
 import java.util.List;
 import org.ray.api.id.UniqueId;
-import org.ray.api.runtime.DefaultRayRuntimeFactory;
 import org.ray.api.runtime.RayRuntime;
 import org.ray.api.runtime.RayRuntimeFactory;
 
@@ -17,7 +16,14 @@ public final class Ray extends RayCall {
    * Initialize Ray runtime with the default runtime implementation.
    */
   public static void init() {
-    init(new DefaultRayRuntimeFactory());
+    try {
+      Class clz = Class.forName("org.ray.runtime.DefaultRayRuntimeFactory");
+      RayRuntimeFactory factory = (RayRuntimeFactory) clz.newInstance();
+      init(factory);
+    } catch (Exception e) {
+      throw new RuntimeException("Failed to initialize Ray runtime.", e);
+    }
+
   }
 
   /**

java/api/src/main/java/org/ray/api/runtime/DefaultRayRuntimeFactory.java

@@ -1,22 +0,0 @@
-package org.ray.api.runtime;
-
-import java.lang.reflect.Method;
-
-/**
- * The default Ray runtime factory. It produces an instance of AbstractRayRuntime.
- */
-public class DefaultRayRuntimeFactory implements RayRuntimeFactory {
-
-  @Override
-  public RayRuntime createRayRuntime() {
-    try {
-      Method m = Class.forName("org.ray.runtime.AbstractRayRuntime").getDeclaredMethod("init");
-      m.setAccessible(true);
-      RayRuntime runtime = (RayRuntime) m.invoke(null);
-      m.setAccessible(false);
-      return runtime;
-    } catch (Exception e) {
-      throw new RuntimeException("Failed to initialize ray runtime", e);
-    }
-  }
-}