[#495] allow override of redis options for env=test

This change allows developers to override redis options for testing by using the test.redis keyspace in the local `etc/flapjack_config.yaml` file.

Previously, spec and cucumber tests had redis options hardcoded as `{ db => 14, :driver => :ruby }`.  

I noticed because I had an old (2.2) redis running on the default port.  

Changes:

* default test values for when no config file is found.
  * must have `:driver => :ruby` to avoid `evma_connect_to_server` errors
  * travis-ci redis-server is on `localhost:6379`, version 2.8 for ruby projects.
  * example yaml must use 'db' key rather than 'database',
    constrained by allowed key processing in `for_redis()`.
    from `flapjack/configuration`.
* adds `:driver` to allowed key processing in `for_redis()`.
* spec_helper change ends up reading the config before every redis test, which is a waste.  My spec/ruby skills were not up to the task of sharing the data from a `config.before(:suite)` test
* `features/support/env.rb` contains redis config for cucumber acceptance tests