[mary_tts] multiple bug fixes

Fixing #52:
* Enabling selection of `en_GB` locale
* Setting `en_GB` as default locale because the default language is part of that

Fixing #50:
* removed autostart from action server

Fixing #53:
* Adding correct permissions to marytts-5.0 install target

Additional bugfix:
* If an empty string is published a warning will be displayed and the service or action server reports failure. This prevents mary from crashing when trying to say nothing.