framework/execution: Move start/stop calls to the TargetManager to Runner

Instead of starting and stopping the target from inside the Context,
move the calls to the Runner. This allows for the any initilizaton
errors to be dealt with as part of the job and retried as
appropriate.

wa/framework/execution.py

@@ -142,13 +142,11 @@ class ExecutionContext(object):
         job_output = init_job_output(self.run_output, self.current_job)
         self.current_job.set_output(job_output)
         self.update_job_state(self.current_job)
-        self.tm.start()
         return self.current_job
 
     def end_job(self):
         if not self.current_job:
             raise RuntimeError('No jobs in progress')
-        self.tm.stop()
         self.completed_jobs.append(self.current_job)
         self.update_job_state(self.current_job)
         self.output.write_result()
@@ -478,6 +476,7 @@ class Runner(object):
 
         try:
             log.indent()
+            context.tm.start()
             self.do_run_job(job, context)
             job.set_status(Status.OK)
         except (Exception, KeyboardInterrupt) as e: # pylint: disable=broad-except
@@ -495,6 +494,7 @@ class Runner(object):
         finally:
             self.logger.info('Completing job {}'.format(job.id))
             self.send(signal.JOB_COMPLETED)
+            context.tm.stop()
             context.end_job()
 
             log.dedent()