Fix novaclient 'launch' server action

The 'launch' server action was previously calling the server manager
'launch' method using positional arguments. With the addition of
the availability_zone parameter, avalability_zone was being assigned
the value of num_instances and num_instances was being assigned None,
resulting in the gridcentric service returning an error.

Change-Id: I421531ce1742a12348f1f466d904478d85b03003