job_coordinator: add the last_stage_task call to the can_finish
job_coordinator: add the last_stage_task call to the can_finish

Previously, we were using the 'and' operator which does not short-circuit and
evaluates the (potentially expensive) last_stage_task function.
With this commit, we only call this function when there is no running tasks.

pombredanne/disco,simudream/disco,pooya/disco,discoproject/disco,ktkt2009/disco,ErikDubbelboer/disco,ktkt2009/disco,beni55/disco,mwilliams3/disco,mwilliams3/disco,seabirdzh/disco,discoproject/disco,seabirdzh/disco,simudream/disco,ktkt2009/disco,pombredanne/disco,ErikDubbelboer/disco,pooya/disco,pooya/disco,oldmantaiter/disco,seabirdzh/disco,oldmantaiter/disco,simudream/disco,pombredanne/disco,oldmantaiter/disco,ktkt2009/disco,discoproject/disco,mwilliams3/disco,beni55/disco,beni55/disco,simudream/disco,pooya/disco,seabirdzh/disco,pombredanne/disco,ErikDubbelboer/disco,seabirdzh/disco,mozilla/disco,simudream/disco,mozilla/disco,ktkt2009/disco,ErikDubbelboer/disco,beni55/disco,ErikDubbelboer/disco,mwilliams3/disco,discoproject/disco,discoproject/disco,oldmantaiter/disco,mozilla/disco,mozilla/disco,oldmantaiter/disco,beni55/disco,pombredanne/disco,mwilliams3/disco