Move getEnvDefault to a util package

So that we can re-use it for tests. I did consider moving the settings
variables themselves, but I think it makes sense to confine them to the main
function/package, at the cost of duplicating the defaults in the tests.
Use AMQP_ADDRESS and REDIS_ADDRESS in tests

So that I can run the integration tests against Redis and RabbitMQ running
in virtual machines rather than my host machine e.g.

    REDIS_ADDRESS="10.3.0.50:6379" AMQP_ADDRESS="amqp://test:test@10.3.0.70:5672" make
Merge pull request #4 from alphagov/tests_amqp_redis_addresses

Use AMQP_ADDRESS and REDIS_ADDRESS in tests