(Feature) Add support for passing parameters via env vars

[phahulin]

• What is it? (leave one option)

  • New (Feature) request (i.e. some nice-to-have feature is missing)

• What problem have you encountered (provide steps to reproduce if possible) / What feature are you talking about? At the present moment private parameters (lob api key, signer keys) for the server should be set in web-dapps/server-config-private.js. Since most of them are plain text, it would be nice to support passing them via env variables and .env files

• What is expected behaviour / What is proposed feature?

[phahulin]

There doesn't seem to be a convenient way to convert the whole server-config-private.js to env vars, because it can contain a json-like structures. Storing contract address in env var was done in https://github.com/poanetwork/poa-popa/pull/98