* fix environment_vars timing bug for EnvString substitution
FRIGATE_ENV_VARS is populated at module import time but was never
updated when environment_vars config section set new vars into
os.environ. This meant HA OS users setting FRIGATE_* vars via
environment_vars could not use them in EnvString fields.
* add EnvString support to MQTT and ONVIF host fields
* add tests
* docs
* update reference config
This supports systemd credentials, see https://systemd.io/CREDENTIALS/.
Default to `/run/secrets` (the Docker Secrets dir) for backwards
compatibility.