add more specific actions to request job enable and immediate start

issuing a job-start does not immediately start a job. It only *requests* a start action. The result is that the scheduler, after successfully fetching the update job document, schedules the job according to the provided scheduling information.
It would be more reasonable to have two ways to "activate" a job: one which simpli "enables" the job, and results in the job being scheduled accordin to the scheduling informations, and one which immediately starts a one-shot execution of a job, regardless of the scheduling information.
So the new actions for the python-freezerclient (formerly freezer-scheduler) would be:
job-start immediately start a single execution of the job
job-abort aborts a running job (not yet implemented by the scheduler)
job-enable enables the job to run according to the provided scheduling information
job-disable un-schedule a job, marking it as "disabled"