Remove deprecated ENV to make it docker-compose friendly

[wernight]

It looks like it currently uses deprecated environment variables like REDIS_PORT_6379_TCP_ADDR in sentry.conf.py:

redis = env('SENTRY_REDIS_HOST') or (env('REDIS_PORT_6379_TCP_ADDR') and 'redis')

This for example doesn't work when using docker-compose.yml equivalent to the started commands:

version: '2'
services:
  sentry:
    image: sentry
    links:
      - postgres
      - redis
    environment:
      - SENTRY_SECRET_KEY=<generated-secret>
  redis:
    image: redis
  postgres:
    image: postgres
    environment:
      - POSTGRES_USER=sentry
      - POSTGRES_PASSWORD=secret

$ docker-compose run --rm sentry upgrade
sentry.runner.importer.ConfigurationError: Exception: Error: REDIS_PORT_6379_TCP_ADDR (or SENTRY_REDIS_HOST) is undefined, did you forget to `--link` a redis container?

Idea for a fix (but needs review):

redis = env('SENTRY_REDIS_HOST', 'redis')

Same for other similar variables and then it works (tested by specifying SENTRY_REDIS_HOST and SENTRY_POSTGRES_HOST but also some that are not shared like POSTGRES_ENV_POSTGRES_PASSWORD and POSTGRES_ENV_POSTGRES_USERNAME and that makes a lot more sense).

[mattrobenolt]

What's wrong with just saying SENTRY_REDIS_HOST=redis? I would rather not make assumptions about connecting to a DNS record of simply "redis" automatically. That's potentially insecure and incorrect. It should be an explicit decision.

[wernight]

Ok, may be improve the error when one provides SENTRY_REDIS_HOST but didn't specify SENTRY_POSTGRES_HOST it fails in Django without talking about missing that environment variable.