support google tts mod

lib/tasks/tts-task.js

@@ -68,6 +68,7 @@ class TtsTask extends Task {
       tts_engine: vendor,
       tts_voice: voice,
       cache_speech_handles: 1,
+      GOOGLE_TTS_APPLICATION_CREDENTIALS: JSON.stringify(credentials.credentials)
     }).catch((err) => this.logger.info({err}, `${this.name}: Error setting tts_engine on endpoint`));
 
     if (!preCache) this.logger.info({vendor, language, voice, model}, `${this.name}:exec`);