Explicit parameter generation syntax in templates

- Add "generate" field to specify generator to be used
- Make "value" field the input value for the generator
- Remove parameter "type" field, it's always "string"
The above change is to make "api" type compatible with "v1beta1" type. Or do we consider "v1beta1" objects to be locked in their current state now?

@mfojtik @soltysh Fyi, as we discussed.