In an effort to make jobs more self contained, and given that dry-run
jobs created temporary content that is not supposed to be kept after
the job is run, this introduces the following changes:

 * Jobs with "dry-run" enabled will now, by default, cleanup their
   own data.  Basically, this means that the "base log directory"
   created during job initialization, and that for non "dry-run"
   jobs is usually "~/avocado/job-results", will also be removed
   at the job's "cleanup()" phase.

 * A new "--dry-run-no-cleanup" command line option, that only makes
   sense when jobs with dry-run enabled start to clean up data for
   themselves

 * Because some result plugins will, by default, write to the job
   results directory, results plugins are now run before a job cleanup
   is done.
Signed-off-by: NCleber Rosa <crosa@redhat.com>