Allow configuration of Redis DB and cache DB

Hardcoding the Redis DB and Redis Caching DB to 0 and 2 in
`config/database.yml` makes an unsafe assumption that Discourse is the
only application using that install of redis-server. Instead of forcing
users to undergo yet another form of configuration, allow Discourse
admins a nicer way to configure the Redis databases used.

Signed-off-by: David Celis <me@davidcel.is>