don't use `UrlFetchTransport` in App Engine Flex environment

[eddavisson]

The decision of whether to use UrlFetchTransport instead of NetHttpTransport is currently based on the presence of the com.google.appengine.application.id system property. This can result in UrlFetchTranport being used in the App Engine Flex environment (at least in compat mode).

One possible approach is:

if (System.getProperty("com.google.appengine.application.id") != null
    // Rule out App Engine Flex compat mode.
    && System.getenv("GAE_VM") == null
    // Rule out App Engine Flex.
    && System.getenv("GCLOUD_PROJECT" ) == null) {
  // use UrlFetchTransport
}

https://github.com/GoogleCloudPlatform/google-cloud-java/blob/e5f90467fe7490759482e6a8bc62b1246318a7f0/google-cloud-core/src/main/java/com/google/cloud/HttpServiceOptions.java#L61

https://github.com/GoogleCloudPlatform/google-cloud-java/blob/0d9d1b3ce548aa5335e23b8c2ec0464df8002d9f/google-cloud-core/src/main/java/com/google/cloud/ServiceOptions.java#L318

[eddavisson]

Note that a customer seems to be hitting this issue in http://stackoverflow.com/questions/40830727/why-does-datastore-produce-timeout-while-fetching-url.