Make possible to configure `host:port` from environment variables without quotes.
Prior to this change, when overriding a `host:port` config value from environment variable, one has to quote it as:
`env EMQX_BRIDGES__MQTT__XYZ__SERVER='"localhost:1883"'`.
Now it's possible to set it without quote as `env EMQX_BRIDGES__MQTT__XYZ__SERVER='localhost:1883'`.