Fix baremetal VIFDriver

Commit Id287f7e661 removed the nova/virt/vif.py file and the VIFDriver
base class, and erroneously stated in its commit message that baremetal
driver did not use it. This patch repeats the work of that earlier patch
by removing the import of nova.virt.vif from baremetal/vif_driver.py.

blueprint general-bare-metal-provisioning-framework

Change-Id: Ifac9510a72a028dc5d3574148f3c4a6410b1d249
Merge "Fix baremetal VIFDriver"