Fix hanging in ipcluster.py when already started or stopped.

Now ipcluster.py start/stop won't hang if the cluster is already started
or stopped.

This also changes the usage. The command line arg passed must actually
call the python function you want. Now we can use the script more generally,
for example, starting an arbitrary number of engines.
    $ python ipcluster.py 'start(n=12)'
Makefile directly calls the python functions.