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
