[2.0b6, UI] Can't add a VLAN over the WebUI.

[Description]
Adding a VLAN over the WebUI results in a nasty traceback in the logs, and an error in the WebUI.

[Reproduce]
1. Go to networks page
2. Add VLAN
3. Input name, VID, Select Fabric.
4. Click on Add

[Output]
You will see "Form requires either a instance or a fabric." in the UI.

And you will the logs has a traceback:

2016-06-06 12:21:13 [-] Error on request (65) vlan.create: Form requires either a instance or a fabric.
        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 904, 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/provisioningserver/utils/twisted.py", line 937, in callInContext
            return func(*args, **kwargs)
          File "/usr/lib/python3/dist-packages/maasserver/utils/orm.py", line 516, in call_within_transaction
            return func_outside_txn(*args, **kwargs)
          File "/usr/lib/python3/dist-packages/maasserver/utils/orm.py", line 343, 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 368, in create
            form = form_class(data=data)
          File "/usr/lib/python3/dist-packages/maasserver/forms_vlan.py", line 41, in __init__
            raise ValueError("Form requires either a instance or a fabric.")
        builtins.ValueError: Form requires either a instance or a fabric.