Create key manager interface

This interface provides a thin wrapper around an underlying key management
implementation such as Barbican or a KMIP server. The key manager interface is
used by the volume encryption code to retrieve keys for volumes.

Implements: blueprint encrypt-cinder-volumes
Change-Id: I9b0dcb7d648ee6809185c71ba457c8a8a6c90d50
SecurityImpact
Merge "Create key manager interface"