[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/example.conf

@@ -0,0 +1,29 @@
+# This is an example ray config file.
+# To use this file, copy it to your classpath and rename it to 'ray.conf'.
+
+# For all available config items and default values,
+# see 'java/runtime/src/main/resources/ray.default.conf'.
+# For config file format, see 'https://github.com/lightbend/config/blob/master/HOCON.md'.
+
+ray {
+  // This is the path to the directory where Ray is installed, e.g.,
+  // something like /home/ubmutu/ray. This can be an absolute path or
+  // a relative path from the current working directory.
+  home = "/path/to/your/ray/home"
+
+  // Run mode, available options are:
+  //
+  // `SINGLE_PROCESS`: Ray is running in one single Java process, without Raylet backend,
+  //                   object store, and GCS. It's useful for debug.
+  // `CLUSTER`: Ray is running on one or more nodes, with multiple processes.
+  run-mode = CLUSTER
+
+  // Available resources on this node.
+  resources: "CPU:4,GPU:0"
+
+  // The address of the redis server to connect, in format `ip:port`.
+  // If not provided, Ray processes will be started locally, including
+  // Redis server, Raylet and object store.
+  redis.address = ""
+
+}