Config does not have a unique index on name

Though m.models.config.Config is intended to have only one value per configuration item name, there is no unique key to enforce this. This means that races can happen in set_config() because it uses get_or_create(). No error is raised by this race until it comes time to retrieve the configuration item:

Traceback (most recent call last):
  File "/usr/lib/python2.7/threading.py", line 783, in __bootstrap
    self.__bootstrap_inner()
  File "/usr/lib/python2.7/threading.py", line 810, in __bootstrap_inner
    self.run()
  File "/usr/lib/python2.7/threading.py", line 763, in run
    self.__target(*self.__args, **self.__kwargs)
--- <exception caught here> ---
  File "/usr/lib/python2.7/dist-packages/twisted/python/threadpool.py", line 191, in _worker
    result = context.call(ctx, function, *args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/twisted/python/context.py", line 118, in callWithContext
    return self.currentContext().callWithContext(ctx, func, *args, **kw)
  File "/usr/lib/python2.7/dist-packages/twisted/python/context.py", line 81, in callWithContext
    return func(*args,**kw)
  File ".../src/provisioningserver/utils/twisted.py", line 143, in wrapper
    return func(*args, **kwargs)
  File ".../src/maasserver/utils/async.py", line 153, in call_within_transaction
    return func(*args, **kwargs)
  File ".../src/maasserver/security.py", line 90, in get_shared_secret
    secret_in_db_hex = Config.objects.get_config("rpc_shared_secret")
  File ".../src/maasserver/models/config.py", line 95, in get_config
    return self.get(name=name).value
  File "/usr/lib/python2.7/dist-packages/django/db/models/manager.py", line 151, in get
    return self.get_queryset().get(*args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/django/db/models/query.py", line 310, in get
    (self.model._meta.object_name, num))
maasserver.models.config.MultipleObjectsReturned: get() returned more than one Config -- it returned 2!