Describe available test resources in a YAML file

Tempest configuration contains a mixture of flags that defined how tempest should behave, and ids and names pointing to test resources to be used.
This has several drawbacks: (1) configuring tempest requires a running cloud as we use IDs of test resources (2) using different resources in different tests requires a proliferation of configuration options (3) configuration file is huge and difficult to handle

We shall move all test resources to a dedicated YAML file, and thus allow for:
- tempest configuration to happen with the same amount of configuration a deployer system uses to configure the OpenStack services
- tests can select resources to be used by logical name or properties (e.g. use whatever image that would fit in "smallest" flavor), or run against all combinations of certain resources (via testscenarios).
Provide a tool to generate such YAML file