Fix/multiple playback thread start

[NeonJarbas]

exception calling callback for <Future at 0x7f8d5527c0 state=finished raised RuntimeError>
Traceback (most recent call last):
  File "/usr/lib/python3.8/concurrent/futures/_base.py", line 328, in _invoke_callbacks
    callback(self)
  File "/home/mycroft/.local/lib/python3.8/site-packages/pyee/_executor.py", line 56, in _callback
    self.emit('error', exc)
  File "/home/mycroft/.local/lib/python3.8/site-packages/pyee/_base.py", line 116, in emit
    self._emit_handle_potential_error(event, args[0] if args else None)
  File "/home/mycroft/.local/lib/python3.8/site-packages/pyee/_base.py", line 86, in _emit_handle_potential_error
    raise error
  File "/usr/lib/python3.8/concurrent/futures/thread.py", line 57, in run
    result = self.fn(*self.args, **self.kwargs)
  File "/home/mycroft/.local/lib/python3.8/site-packages/mycroft/audio/speech.py", line 63, in handle_speak
    mute_and_speak(utterance, ident, listen)
  File "/home/mycroft/.local/lib/python3.8/site-packages/mycroft/audio/speech.py", line 89, in mute_and_speak
    tts.init(bus)
  File "/home/mycroft/.local/lib/python3.8/site-packages/ovos_plugin_manager/templates/tts.py", line 445, in init
    TTS.playback.start()
  File "/usr/lib/python3.8/threading.py", line 848, in start
    raise RuntimeError("threads can only be started once")
RuntimeError: threads can only be started once

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  Misses         ?    1788           
  Partials       ?       0           

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