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

* default test values for when no config file is found.
  * must have :driver => :ruby
  * travis-ci redis-server is on localhost:6379
  * 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