Use the actual current database configuration

Rails.configuration.database_configuration returns config found in the
database_configuration file, rather than the currently set config.
This causes sporadic issues during parallel test runs because our
ParallelTestRunner customises the config per-fork. Instead of loading
the config from the YAML file, we grab the config that is  currently
set one on ActiveRecord::Base.
Merge pull request #661 from alphagov/parallel-testing-fix

Use the actual current database configuration