In case creating a VM fails before the OP_INSTANCE_CREATE job is
enqueued in Ganeti, the VM and its NICs must be deleted, and the
resources must be released from Quotaholder, which is exactly what is
done when a VM is deleted from Ganeti. Instead of duplicating the code,
this commit uses the same function that is used by snf-dispatcher, by
mocking an successfully OP_INSTANCE_REMOVE.

Introduce assertMethodNotAllowed method in BaseAPITest class. The method
asserts that provided response is a valid ``Method not allowed`` API response.

Define assertGreater, assertIn, assertRaises in snf_django.utils.testing

Mock 'resolve_commissions' call.

Create 'mocked_quotaholder' context manager to bypass commission and use
it in Cyclades tests. Also this commit does some various fixes to other
tests.

New synnefo.util.testing lib added. Currently including helper decorator and
context processor to override django settings.