refactor/tts_mode

[NeonJarbas]

allow audio service to load even if there are no audio backend plugins

We always want the voice assistant to be able to speak, even if skills can not play music

to test uninstall all audio backend plugins and run the audio service. confirm you still have TTS

[codecov]

Codecov Report

Merging #128 (b536de8) into dev (6ceb058) will increase coverage by 2.94%.
The diff coverage is 34.87%.

@@            Coverage Diff             @@
##              dev     #128      +/-   ##
==========================================
+ Coverage   50.35%   53.29%   +2.94%     
==========================================
  Files         119      149      +30     
  Lines       10077     9080     -997     
==========================================
- Hits         5074     4839     -235     
+ Misses       5003     4241     -762     

Impacted Files	Coverage Δ
mycroft/api/__init__.py	30.24% <0.00%> (-47.99%)	⬇️
mycroft/audio/__main__.py	0.00% <0.00%> (ø)
mycroft/client/enclosure/__main__.py	0.00% <0.00%> (ø)
mycroft/client/enclosure/mark1/arduino.py	0.00% <0.00%> (ø)
mycroft/client/enclosure/mark1/eyes.py	0.00% <0.00%> (ø)
mycroft/client/enclosure/mark1/mouth.py	0.00% <0.00%> (ø)
mycroft/client/speech/__main__.py	0.00% <0.00%> (ø)
mycroft/client/speech/hotword_factory.py	0.00% <0.00%> (-88.89%)	⬇️
mycroft/client/speech/service.py	0.00% <0.00%> (ø)
mycroft/client/speech/silence.py	0.00% <0.00%> (-42.86%)	⬇️
... and 87 more

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