Call start_file_logging inside avocado.job.

Call start_file_logging inside avocado.job, so Test Results
just need to use output_manager/stream object.

Signed-off-by: Ruda Moura <rmoura@redhat.com>
avocado.plugins.vm: Use self.stream.debuglog.

Signed-off-by: Ruda Moura <rmoura@redhat.com>
avocado.plugins.jsonresult: Use self.stream.debuglog.

Signed-off-by: Ruda Moura <rmoura@redhat.com>
Merge pull request #74 from ruda/job_owns_start_file_logging

Job  class owns execution of start_file_logging()