avocado.job: Make Job.run() to return a status int

Make the run() method in the Job class to return a
status int. Take the opportunity and create a core
status library, that contains a mapping between
status words to booleans, that contain a cut and
dry answer to 'this operation passed or failed?'
question.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>