Make application secrets more flexible

[jakthom]

Currently there's one way of providing secrets to honeypot - a file titled config.yml in the same directory as the binary. This is obviously non-ideal.

Options (probably all of them are ideal)

1. Add env var specifying config file location override
2. Add env-var-based config overrides: LEVEL1_LEVEL2_VARNAME
3. Add default config.yml to the docker build

[jakthom]

Added default config to docker build and env-var-based config file override. It's working well.

[jakthom]

The second one is genius (thanks benthos), but will come back to implementing it later as asked.