Solum DU functional tests

This blueprint considers configuration and execution of functional 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 functional 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 functional tests pass or fail
- Ability to log test results in a persistent storage

In addition to above we also need to consider the issue of how and where to spin up the resources required for doing such testing.
For instance, an assembly may require a compute instance and a database instance. For end-to-end functional testing Solum would need to spin up these resources and take them down afterwards.
For 'how' to spin up the resources, we could just create a 'test' Heat stack (similar to what we are currently doing for actual assembly deployment). We would need to destroy this stack once functional tests are run.
For 'where' to spin up the resources, most probably we would go with using the 'test environment' (so the environment work is a dependency for this).