Expose internal config parameters for starting Ray (#3246)

## What do these changes do?

This PR exposes the CL option for using a config parameter. This is important for certain tests (i.e., FT tests that removing nodes) to run quickly.

Note that this is bad practice and should be replaced with GFLAGS or some equivalent as soon as possible.

#3239 depends on this.

TODO:
 - [x] Add documentation to method arguments before merging.
 - [x] Add test to verify this works?

## Related issue number

python/ray/worker.py

@@ -1293,7 +1293,8 @@ def _init(address_info=None,
           driver_id=None,
           plasma_store_socket_name=None,
           raylet_socket_name=None,
-          temp_dir=None):
+          temp_dir=None,
+          _internal_config=None):
     """Helper method to connect to an existing Ray cluster or start a new one.
 
     This method handles two cases. Either a Ray cluster already exists and we
@@ -1355,6 +1356,8 @@ def _init(address_info=None,
             used by the raylet process.
         temp_dir (str): If provided, it will specify the root temporary
             directory for the Ray process.
+        _internal_config (str): JSON configuration for overriding
+            RayConfig defaults. For testing purposes ONLY.
 
     Returns:
         Address information about the started processes.
@@ -1427,7 +1430,8 @@ def _init(address_info=None,
             include_webui=include_webui,
             plasma_store_socket_name=plasma_store_socket_name,
             raylet_socket_name=raylet_socket_name,
-            temp_dir=temp_dir)
+            temp_dir=temp_dir,
+            _internal_config=_internal_config)
     else:
         if redis_address is None:
             raise Exception("When connecting to an existing cluster, "
@@ -1468,6 +1472,9 @@ def _init(address_info=None,
         if raylet_socket_name is not None:
             raise Exception("When connecting to an existing cluster, "
                             "raylet_socket_name must not be provided.")
+        if _internal_config is not None:
+            raise Exception("When connecting to an existing cluster, "
+                            "_internal_config must not be provided.")
 
         # Get the node IP address if one is not provided.
         if node_ip_address is None:
@@ -1530,6 +1537,7 @@ def init(redis_address=None,
          plasma_store_socket_name=None,
          raylet_socket_name=None,
          temp_dir=None,
+         _internal_config=None,
          use_raylet=None):
     """Connect to an existing Ray cluster or start one and connect to it.
 
@@ -1601,6 +1609,8 @@ def init(redis_address=None,
             used by the raylet process.
         temp_dir (str): If provided, it will specify the root temporary
             directory for the Ray process.
+        _internal_config (str): JSON configuration for overriding
+            RayConfig defaults. For testing purposes ONLY.
 
     Returns:
         Address information about the started processes.
@@ -1658,7 +1668,8 @@ def init(redis_address=None,
         driver_id=driver_id,
         plasma_store_socket_name=plasma_store_socket_name,
         raylet_socket_name=raylet_socket_name,
-        temp_dir=temp_dir)
+        temp_dir=temp_dir,
+        _internal_config=_internal_config)
     for hook in _post_init_hooks:
         hook()
     return ret