Fixes the destroy() method for the Docker virt driver

* Added the missing "context" argument in nova/virt/driver.py
* Added the missing argument in the test_virt_drivers.destroy() as well so it
will make sure none of the other drivers will miss it.

Change-Id: I083b935da95c28ad4f9879036ef2bf7c1fbbb58e
Closes-Bug: #1244829
Merge "Fixes the destroy() method for the Docker virt driver"