Use common test for chef_db integration tests

This PR depends on https://github.com/opscode/chef_db/pull/40

Using common test for integration tests is a good fit because of the directory management and log reports that common test provides -- and also in terms of the testing strategy it encourages a more integration-ey rather than unit-ey approach.

One of the nice features of this setup is that a new postgres instance is started for each test. This will make it easier to run the tests as part of CI.

ping @marcparadise @manderson26 @hosh @oferrigni @sdelano @christophermaier