Override appengine_v1::VersionsClient Authentication Defaults

 [](std::string const& keyfile) {
 auto is = std::ifstream(keyfile);
 is.exceptions(std::ios::badbit); // Minimal error handling in examples
 auto contents = std::string(std::istreambuf_iterator<char>(is.rdbuf()), {});
 auto options =
 google::cloud::Options{}.set<google::cloud::UnifiedCredentialsOption>(
 google::cloud::MakeServiceAccountCredentials(contents));
 return google::cloud::appengine_v1::VersionsClient(
 google::cloud::appengine_v1::MakeVersionsConnection(options));
 }