Handle config files with name `config.jsonnet`, `config.json` and
`config.yml` and take the first from the repository in this order
For a jsonnet file execute it and use the generated output as the config
The current config format was thought for future extensions for reusing runtimes
and job definitions adding some parameters.
After a lot of thoughts this looks like a complex approach: the final result
will be a sort of templating without a lot of powers.
Other approach like external templating should be an alternative but I really
don't think templating yaml is the way to go.
A much better approach will to just use jsonnet when we need to create matrix
runs and a lot of other use cases.
So just make the config a simple yaml/json. User can generate their config using
any preferred tool and in future we'll leverage jsonnet automated parsing and
provide a lot of jsonnet based examples for most use cases.
Main changes:
* Runs are now an array and not a map. The run name is in the Name field
* Tasks are now an array and not a map. The task name is in the Name field
* Use https://github.com/ghodss/yaml so we'll use json struct tags and unmarshall functions
