freezer api support for jobs

One fundamental role for the api is to store, search and retrieve instruction for the freezer client to run.
This is a sort of "recipe" that the freezer scheduler, running on the clients, will get and feed to the actual freezerc for execution.
We can call this recipe a "job".

The distinction between "configuration-file" and "action" is unnecessary. All that is needed is the possibility to add scheduling information to the job.

A single api endpoint for jobs simplifies access and use.