Add delete_net_interface function

This commit adds a new function that deletes a specific
tap device. This bug fix is targeted for Havana.

fixes: bug #1138408
(cherry picked from commit 9c86567201b9cf51e1ed0b0c55a9d614d4988dce)

Conflicts:
	nova/network/linux_net.py

Change-Id: I26be5d94550db35358caae642fb9000953346826