Add system env to all configs

[nikolauska]

This merge adds possibility to use system environment variables on all configs. Having only username and password makes it harder to change other settings when building released package with distillery.

[nikolauska]

I just realized it's not this easy, I'll add tests and fix this to properly work

[kdisneur]

While you're at changing the code, what do you think of re-calling the function using the interpolated value? It could make it less error-prone if we don't duplicate the configuration keys.

Example (always easier 😸 )

defp to_gen_smtp_server_config({:server, {:system, var}}, config) do
  to_gen_smtp_server_config({:server, System.get_env(var)}, config)
end

[nikolauska]

it could be easier to make it generic like this

defp to_gen_smtp_server_config({conf, {:system, var}}, config) do
  to_gen_smtp_server_config({conf, System.get_env(var)}, config)
end

and then leave handling different types of configs in specific config for example tls requires atom and System.get_env returns string (this is actually why I realized it wasn't this easy)

[nikolauska]

I have now reworked settings system and added more validation to make sure values are always correct type. I've also added test for all configs variables to validate that system values are correctly transformed from string to proper format.

[tgautier]

Seems good to me.

Could you just rebase this to one commit before we accept your PR?

Thanks!

[nikolauska]

It's now rebased to one commit