[#58501092] Replace CLI args with env vars

This will make it easier to decouple configuration, deployment and process
management. Previous behaviour is maintained:

- All options have sensible defaults.
- There is no explicit syntax checking of values.

Help text has been removed. Will reinstate that in some form.
[#58501092] Help/usage for available env vars

Reinstate flag, in a rather simple capacity, to document the available
environment variables. There's some duplication here. Particularly the
default values. But I'm not sure that's such a big deal.
Merge pull request #18 from alphagov/58501092-config_env_vars

[Delivers #58501092] Move config to env vars