Add a local configuration source and a non-environmental ENV config source

Summary:
See discussion in T2221. Before we can move configuration to the database, we have a bootstrapping problem: we need database credentials to live somewhere if we can't guess them (and we can only really guess localhost / root / no password).

Some options for this are:

• Have them live in ENV variables.
  • These are often somewhat unfamiliar to users.
  • Scripts would become a huge pain -- you'd have to dump a bunch of stuff into ENV.
  • Some environments have limited ability to set ENV vars.
  • SSH is also a pain.
• Have them live in a normal config file.
  • This probably isn't really too awful, but:
  • Since we deploy/upgrade with git, we can't currently let them edit a file which already exists, or their working copy will become dirty.
  • So they have to copy or create a file, then edit it.
  • The biggest issue I have with this is that it will be difficult to give specific, easily-followed directions from Setup. The instructions need to be like "Copy template.conf.php to real.conf.php, then edit these keys: x, y, z". This isn't as easy to follow as "run script Y".
• Have them live in an abnormal config file with script access (this diff).
  • I think this is a little better than a normal config file, because we can tell users 'run phabricator/bin/config set mysql.user phabricator' and such, which is easier to follow than editing a config file.

I think this is only a marginal improvement over a normal config file and am open to arguments against this approach, but I think it will be a little easier for users to deal with than a normal config file. In most cases they should only need to store three values in this file -- db user/host/pass -- since once we have those we can bootstrap everything else. Normal config files also aren't going away for more advanced users, we're just offering a simple alternative for most users.

This also adds an ENVIRONMENT file as an alternative to PHABRICATOR_ENV. This is just a simple way to specify the environment if you don't have convenient access to env vars.

Test Plan: Ran config set x y, verified writes. Wrote to ENVIRONMENT, ran PHABRICATOR_ENV= ./bin/repository.

Reviewers: btrahan, vrana, codeblock

Reviewed By: codeblock

CC: aran

Maniphest Tasks: T2221

Differential Revision: https://secure.phabricator.com/D4294