Configure using environment variables

[rwb27]

#73 introduced the ability to configure a server using a file or command line flags. However, if we're running from a server process using ASGI, rather than directly from the command line using uvicorn, we won't get command line arguments. I propose we swap the command line flags for environment variables, which can be used more reliably to set the parameters. Specifically, something like LABTHINGS_CONFIG_FILE or LABTHINGS_CONFIG_JSON would be best. It might be simpler just to force the use of a file - temp files can always be used for testing, if necessary: that was the main motivation for the ability to supply json directly.