Allow for noninteractive startup when using API authentication

Created by: ericflo

I've been working on automating my setup into a kubernetes cluster, and I want it to be secure even though it's automated. Currently Sia asks for a password in an interactive way, which is a showstopper for this project.

This patch adds the environment variable SIA_API_PASSWORD. On startup if it's detected to be non-empty, that will be used instead of prompting the user. Just as with SIA_WALLET_PASSWORD it prints a line indicating that it has picked up the password from the environment -- so nobody should be caught off-guard.