Make system.secret-key configurable via environment

[glensc]

Problem Statement

I would like to keep sentry/config.yml in a git repository and share it with two instances: https://sentry.test.onpremise.example.com and https://sentry.production.onpremise.example.com. committing value of system.secret-key is not appropriate there.

Solution Brainstorm

There's sentry/sentry.conf.py python code, which imports a bunch of stuff from the environment, why can't that key also be there?

An alternative would be to place the secret key to a file, for example: sentry/secret-key

[aldy505]

@BYK @hubertdeng123 do you think this is something that's good to work on? For me, this is easy enough though. Will be a similar work with @williamdes's PR #3664