Increase default max nginx upload size to 2GB

In order to support new versions of the Java buildpack, and CNBs in the
future, we should increase the max uploadable file size to 2GB. E.g.:
- cloudfoundry/java-buildpack#1035

[#186470719](https://www.pivotaltracker.com/story/show/186470719)

jobs/cloud_controller_ng/spec

@@ -768,7 +768,7 @@ properties:
     default: "15M"
     description: "Maximum body size for nginx"
   cc.app_bits_max_body_size:
-    default: "1536M"
+    default: "2048M"
     description: "Maximum body size for nginx bits uploads"
 
   cc.default_app_log_rate_limit_in_bytes_per_second: