division by zero calculating usage_precentage

[Blake Rouse]

This occurs when you have a subnet that has a prefixlen of 32.

2016-03-03 14:15:14 [-] Error on request (37) subnet.list: float division by zero
 Traceback (most recent call last):
   File "/usr/lib/python3.5/threading.py", line 862, in run
     self._target(*self._args, **self._kwargs)
   File "/usr/lib/python3/dist-packages/provisioningserver/utils/twisted.py", line 794, in worker
     return target()
   File "/usr/lib/python3/dist-packages/twisted/_threads/_threadworker.py", line 46, in work
     task()
   File "/usr/lib/python3/dist-packages/twisted/_threads/_team.py", line 190, in doWork
     task()
 --- <exception caught here> ---
   File "/usr/lib/python3/dist-packages/twisted/python/threadpool.py", line 246, in inContext
     result = inContext.theWork()
   File "/usr/lib/python3/dist-packages/twisted/python/threadpool.py", line 262, in <lambda>
     inContext.theWork = lambda: context.call(ctx, func, *args, **kw)
   File "/usr/lib/python3/dist-packages/twisted/python/context.py", line 118, in callWithContext
     return self.currentContext().callWithContext(ctx, func, *args, **kw)
   File "/usr/lib/python3/dist-packages/twisted/python/context.py", line 81, in callWithContext
     return func(*args,**kw)
   File "/usr/lib/python3/dist-packages/maasserver/utils/orm.py", line 448, in call_within_transaction
     return func_outside_txn(*args, **kwargs)
   File "/usr/lib/python3/dist-packages/maasserver/utils/orm.py", line 275, in retrier
     return func(*args, **kwargs)
   File "/usr/lib/python3.5/contextlib.py", line 30, in inner
     return func(*args, **kwds)
   File "/usr/lib/python3/dist-packages/maasserver/websockets/base.py", line 327, in list
     for obj in objs
   File "/usr/lib/python3/dist-packages/maasserver/websockets/base.py", line 327, in <listcomp>
     for obj in objs
   File "/usr/lib/python3/dist-packages/maasserver/websockets/base.py", line 194, in full_dehydrate
     return self.dehydrate(obj, data, for_list=for_list)
   File "/usr/lib/python3/dist-packages/maasserver/websockets/handlers/subnet.py", line 36, in dehydrate
     data['statistics'] = metadata.render_json()
   File "/usr/lib/python3/dist-packages/provisioningserver/utils/network.py", line 190, in render_json
     "usage": self.usage_percentage,
   File "/usr/lib/python3/dist-packages/provisioningserver/utils/network.py", line 174, in usage_percentage
     return float(self.num_unavailable) / float(self.total_addresses)
 builtins.ZeroDivisionError: float division by zero