Synchronize the key manager interface with Cinder

This change synchronizes the key manager interface with code that
has been accepted by Cinder. The default key manager (i.e.,
NotImplementedKeyManager) raises NotImplementedError for all
operations. A copy_key method has also been added to the key
manager interface so that keys may be deleted when the objects
that they encrypt (e.g., a volume) are deleted.

Implements blueprint encrypt-cinder-volumes

Change-Id: Ie9ab9578402e87338b6a4bd413bb9f875d3b3eb6
Merge "Synchronize the key manager interface with Cinder"