Solum DU unit tests

This blueprint considers configuration and execution of unit tests in the workflow of creating a DU.

Towards this, we need following capabilities in Solum:

- Ability in the plan definition to control turning on/off running of unit tests
- Ability to detect presence of appropriate test description files in source repo (.drone.yml, .kitchen.yml, etc.)
- Ability to invoke tests in our build workflow
- Ability to execute successive steps in the workflow based on whether unit tests pass or fail
- Ability to log test results in a persistent storage