Feature: make redis password configurable

[hazcod]

We should be able to make the requirepass directive configurable instead of having to modify the redis config.

[hazcod]

Ideally this should be done using confd to not have to run some kind of shell. So:

1. confd loads whitelisted environment variables
2. confd replaces environment variables in confd templates
3. confd generates redis configuration in /tmp/
4. redis starts with config read in /tmp/