Internal server error when trying to stop a node with no power type

Now that bug 1274871 is fixed, it's possible for a Node to exist without a known power type (in effect, manually-power on and off only). MAAS can't handle stopping nodes with no power type set.

To reproduce:
 - Commission a node.
 - Using `maas shell`, set the Node's power type to '':

>>> from maasserver.models.node import Node
>>> n = Node.objects.get(system_id='<system_id>')
>>> n.power_type = ''
>>> n.save()

 - On the Node page, click "Start node" and start the node manually (since it has no power type, MAAS can't start it for you).
 - On the Node page, click "Stop node".

You'll see a white default apache "Internal Server Error" page.

If you try to stop the node through the CLI, you'll see something like this:

ubuntu@ubuntu:~⟫ maas-cli maas node stop node-07307b52-8a72-11e3-970d-000c29376b22
Node power type is unconfigured

Either way, the node stays up in MAAS: essentially there's no way to mark it stopped without using `maas shell` to change the status of the node.

maas.log shows a perfectly cromulent error in both cases:

ERROR 2014-01-31 12:25:46,175 django.request Internal Server Error: /MAAS/nodes/node-07307b52-8a72-11e3-970d-000c29376b22/view/
Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/django/core/handlers/base.py", line 114, in get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/usr/lib/python2.7/dist-packages/django/views/generic/base.py", line 69, in view
    return self.dispatch(request, *args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/maasserver/views/nodes.py", line 453, in dispatch
    return super(NodeView, self).dispatch(*args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/django/views/generic/base.py", line 87, in dispatch
    return handler(request, *args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/django/views/generic/edit.py", line 228, in post
    return super(BaseUpdateView, self).post(request, *args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/django/views/generic/edit.py", line 171, in post
    return self.form_valid(form)
  File "/usr/lib/python2.7/dist-packages/django/views/generic/edit.py", line 147, in form_valid
    self.object = form.save()
  File "/usr/lib/python2.7/dist-packages/maasserver/forms.py", line 529, in save
    message = action.execute(allow_redirect=allow_redirect)
  File "/usr/lib/python2.7/dist-packages/maasserver/node_action.py", line 264, in execute
    self.node.release()
  File "/usr/lib/python2.7/dist-packages/maasserver/models/node.py", line 788, in release
    Node.objects.stop_nodes([self.system_id], self.owner)
  File "/usr/lib/python2.7/dist-packages/maasserver/models/node.py", line 302, in stop_nodes
    node_power_type = node.get_effective_power_type()
  File "/usr/lib/python2.7/dist-packages/maasserver/models/node.py", line 696, in get_effective_power_type
    raise ValueError("Node power type is unconfigured")
ValueError: Node power type is unconfigured

MAAS ought to be able to handle this properly; it handles *starting* nodes with no power type perfectly fine.