helpdesk: Fix issue in tests caused by mock

Helpdesk tests patched methods of the 'logic.backend' without reverting
the patch after the tests. This resulted in module's methods to be globally
patched, affecting other tests. This commit fixes this issue by using
the 'mock.patch' context manager that handles patching within the scope
of a test.
cyclades: Fix bug in unittest

The 'serial' attribute is automatically cleared from the VM model when
the corresponding commission is resolved and so is not available in the
unittest. This commit fixes this issue by not explicitly checking for
the specific serial number.