Increase default max nginx upload size to 2GB (#358)

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: