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

(cherry picked from commit 6ef9262a6aa39b2da8023067386b832a35ecc45c)

Conflicts:

	nova/tests/virt/hyperv/test_hypervapi.py
	nova/tests/virt/vmwareapi/test_vmwareapi.py
Change-Id: Id17a1f691f46816eadfe257d9ac0cc46502318ef
Merge "plug_vif raise NotImplementedError instead of pass" into stable/havana