function getYouTubeAPIKey() {
  try {
    let apiKey = process.env.YOUTUBE_API_KEY;
    if (!apiKey) {
      apiKey = fs.readFileSync('./src/data/youtubeAPIKey.txt', 'utf8');
    }
    return apiKey.trim();
  } catch (ex) {
    gutil.log(' ', 'youtubeAPIKey not found.');
  }
  return null;
}
Gets the YouTube API key.

@return {string} YouTubeAPI key, or null.