[2.1, rev5670] Can't release machines previously deployed/failed deployment/or newly deployed after upgrade to latest trunk

[Andres Rodriguez]

I had the following machines:

a. (1) Deployed
b. (2) Failed deployment
c. (3) Ready

I upgraded to latest trunk rev5670 from a version that didn't use the new 'script' work.

1. After trying to release (a) and (b), I see the issue below (both in UI and traceback).
2. I tried to deploy (3) and then try to release, see the same issue. However, I clicked on 'retry' and it released fine this time.

Steps to reproduce
==================
1. Install MAAS 2.1
2. Deploy machine
3. Upgrade to latest trunk rev5670
4. Release the machine.

==> /var/log/maas/regiond.log <==
2017-01-26 17:19:03 maasserver.websockets.protocol: [critical] Error on request (87) machine.action: {"current_installation_script_set": ["script set instance with id 7 does not exist."]}

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 824, 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 857, in callInContext
    return func(*args, **kwargs)
  File "/usr/lib/python3/dist-packages/provisioningserver/utils/twisted.py", line 225, in wrapper
    result = func(*args, **kwargs)
  File "/usr/lib/python3/dist-packages/maasserver/utils/orm.py", line 685, in call_within_transaction
    return func_outside_txn(*args, **kwargs)
  File "/usr/lib/python3/dist-packages/maasserver/utils/orm.py", line 503, 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/handlers/machine.py", line 720, in action
    return action.execute(**extra_params)
  File "/usr/lib/python3/dist-packages/maasserver/node_action.py", line 395, in execute
    secure_erase=secure_erase, quick_erase=quick_erase)
  File "/usr/lib/python3/dist-packages/maasserver/models/node.py", line 2500, in release_or_erase
    self.release(user, comment)
  File "/usr/lib/python3/dist-packages/maasserver/models/node.py", line 2394, in release
    self._release(user)
  File "/usr/lib/python3/dist-packages/maasserver/models/node.py", line 2465, in _release
    self._finalize_release()
  File "/usr/lib/python3/dist-packages/maasserver/utils/orm.py", line 669, in call_within_transaction
    return func_within_txn(*args, **kwargs)
  File "/usr/lib/python3.5/contextlib.py", line 30, in inner
    return func(*args, **kwds)
  File "/usr/lib/python3/dist-packages/maasserver/models/node.py", line 2483, in _finalize_release
    self.save()
  File "/usr/lib/python3/dist-packages/maasserver/models/node.py", line 1522, in save
    super(Node, self).save(*args, **kwargs)
  File "/usr/lib/python3/dist-packages/maasserver/models/cleansave.py", line 28, in save
    self.full_clean()
  File "/usr/lib/python3/dist-packages/django/db/models/base.py", line 1171, in full_clean
    raise ValidationError(errors)
django.core.exceptions.ValidationError: {'current_installation_script_set': ['script set instance with id 7 does not exist.']}