Internal server error when configuring dynamic range to have fewer than 2 addresses

[Gavin Panella]

1. Edit a cluster interface.
2. Set "DHCP dynamic IP range low value" and "DHCP dynamic IP range high
   value" to the same address.
3. Click "Save interface".
4. Blank page with "Internal server error".

regiond.log contains:

  Traceback (most recent call last):
    File ".../twisted/web/wsgi.py", line 315, in run
      appIterator = self.application(self.environ, self.startResponse)
    File ".../django/core/handlers/wsgi.py", line 206, in __call__
      response = self.get_response(request)
    File ".../maasserver/utils/views.py", line 236, in get_response
      response = get_response(request)
    File ".../maasserver/utils/views.py", line 225, in get_response
      request, get_resolver(None), sys.exc_info())
  --- <exception caught here> ---
    File ".../maasserver/utils/views.py", line 210, in get_response
      return django_get_response(request)
    File ".../maasserver/utils/orm.py", line 357, in __exit__
      self.fire()
    File ".../provisioningserver/utils/twisted.py", line 200, in wrapper
      return func(*args, **kwargs)
    File ".../maasserver/utils/async.py", line 203, in fire
      self._fire_in_reactor(hook).wait()
    File ".../crochet/_eventloop.py", line 219, in wait
      result.raiseException()
    File "<string>", line 2, in raiseException
  exceptions.ValueError: IP sequence must contain at least 2 elements!

This problem should have been discovered during form validation and
rendered as such.

Configuring a dynamic range to contain only a single IP address is a
fairly pathological thing to do, and as such I don't think this is a
particularly serious bug.