plug_vif raise NotImplementedError instead of pass

The Hyper-V, VMware, Docker and PowerVM drivers raise
NotImplementedError from the plug_vifs and unplug_vifs
methods rather than pass.
The compute manager handles the NotImplementedError
case when initializing the host.

Fixed bug 1218593

Change-Id: Id17a1f691f46816eadfe257d9ac0cc46502318ef
Merge "plug_vif raise NotImplementedError instead of pass"