feat(controller): add deis certs:transfer endpoint

[sstarcher]

Certificates are owned by the individual who creates them. If this person leaves the only option is to delete the cert and re-add the cert.

Originally requested from Deis v1 - deis/deis#4576
Similar to app:transfer - deis/deis#4193

[bacongobbler]

For now most administrators have been handling this inside the container using https://deis.com/docs/workflow/troubleshooting/kubectl/:

kubectl --namespace=deis exec -it deis-controller-h6lk6 python manage.py shell
>>> from api.models import Certificate
>>> from django.contrib.auth.models import User
>>> newowner = User.objects.get(username='newowner')
>>> c = Certificate.objects.get(name='mycertname')
>>> c.owner = newowner
>>> c.save()

But I agree that having some form of god-mode visibility and transfer-ability for all resources (including certificates) through the API would be useful.

[Cryptophobia]

This issue was moved to teamhephy/workflow#51