Restore backup database.yml when retesting after interrupt

MSP-11153

Restore the config/database.yml backed up to
config/database.yml.cucumber.bak in the db:config:restore task, which is
made a dependency of the environment rake task so that
config/database.yml is restored before Rails tries to use it in the
environment task.  This specifically, allows for rake cucumber to be
interrupted when the config/database.yml has been moved to
config/database.yml.cucumber.bak and a subsequence rake cucumber to
succeed and restore config/database.yml, but any task that depends on
environment will restore the config/database.yml.