Allow filesystem to be changed on config

Allow to change the filesystem storage on the fly.
For example, you can swap the storage disk with a fake one with
Storage::fake for tests.

config/filesystems.php

@@ -13,7 +13,7 @@ return [
     |
     */
 
-    'default' => 'local',
+    'default' => env('FILESYSTEM_DRIVER', 'local'),
 
     /*
     |--------------------------------------------------------------------------
@@ -26,7 +26,7 @@ return [
     |
     */
 
-    'cloud' => 's3',
+    'cloud' => env('FILESYSTEM_CLOUD', 's3'),
 
     /*
     |--------------------------------------------------------------------------