Rework the multi-instance gnt commands

This patch changes the multi-instance gnt-* commands (gnt-instance
start/stop, gnt-node evacuate/failover) such that the individual
operations are submitted in parallel, ideally improving the speed of the
execution.

The patch does this by abstracting the job set functionality into a new
class in cli.py, that takes care of the job submit, job poll and error
handling.

Reviewed-by: ultrotter