Introduce "Runnable Recipe" resolver

[clebergnu]

Avocado uses a Runnable to describe what is to be executed (usually a test) in a given Task. Runnables (and Tasks) can be defined in JSON files, which are called Recipes (see the examples/nrunner/recipes directory).

These can be loaded and executed with a command such as:

avocado-runner-exec-test runnable-run-recipe
examples/nrunner/recipes/runnables/exec_test_echo_no_newline.json

It may also be useful to run them with an Avocado job. To do that, a "runnable-recipe" resolver is being introduced, which can read from these JSON files and create runtime runnables (which will be added to a job, a suite and turned into a runtime task).