When the app feature 'file-based service bindings' is enabled,
SERVICE_BINDING_ROOT is returned instead of VCAP_SERVICES.

For an app using file-based service bindings the '/env' endpoint (i.e.
GET /v3/apps/:guid/env) returns the following:

  {
    ...
    "system_env_json": {
      "SERVICE_BINDING_ROOT": "/etc/cf-service-bindings"
    },
    ...
  }

The file runtime_environment/system_env_presenter_spec.rb has been
deleted because all the tests it contained are also present in
system_environment/system_env_presenter_spec.rb.

- validate binding names and (credential) keys
- check for duplicate binding names
- check the total bytesize, maximum allowed size is 1MB
- files are added in the following order:
   1. credential keys
   2. VCAP_SERVICES attributes
   3. 'type' and 'provider'
- in case a credential key equals a VCAP_SERVICES attributes or 'type'
  or 'provider', it will be overwritten
- for VCAP_SERVICES attribute names, underscores are replaced by hyphens
- file content is serialized as JSON (non-string objects)

- AppRecipeBuilder -> Diego::Bbs::Models::DesiredLRP
- TaskRecipeBuilder -> Diego::Bbs::Models::TaskDefinition