Fix meetpad audio mute setting

We set the START_AUDIO_MUTE value in our docker compose .env file, but
we didn't pass that value through to the container via the environment
setting in docker-compose.yaml. Fix this so that the jitsi meet config
templating can write out the expected config with the new value.

Change-Id: I3dbebad3ce67a8787ffd31c0db8d9583fe988e50

playbooks/roles/jitsi-meet/files/jitsi-meet-docker/meet-docker-compose.yaml

@@ -42,6 +42,7 @@ services:
             - JIBRI_RECORDER_PASSWORD
             - ENABLE_RECORDING
             - START_VIDEO_MUTED
+            - START_AUDIO_MUTED
 
     # XMPP server
     prosody: