Respect DB_CONNECTION_STRING in integration tests

[nota-ja]

• A short explanation of the proposed change:
  The current implementation of the integration tests ignores DB_CONNECTION_STRING,
  so the tests fail if the database setting is different to the default.
  It will be more convenient to respect DB_CONNECTION_STRING in the integration tests.
• An explanation of the use cases your change solves
  • When running the integration tests with a database having setttings other than the default, you will not get errors
  • When running all the tests with a database having setttings other than the default, you can run them at one time without errors
• I have viewed signed and have submitted the Contributor License Agreement
• I have made this pull request to the master branch
• I have run all the unit tests using bundle exec rake
• I have run CF Acceptance Tests on bosh lite

[cfdreddbot]

Hey nota-ja!

Thanks for submitting this pull request! I'm here to inform the recipients of the pull request that you and the commit authors have already signed the CLA.

[cf-gitbot]

We have created an issue in Pivotal Tracker to manage this:

https://www.pivotaltracker.com/story/show/129608081

The labels on this github issue will be updated when the story is started.