Add key manager implementation with static key

Add key manager implementation with static key

Per feedback received on other patch sets, an example key manager
driver is required to support ephemeral storage encryption and
Cinder volume encryption -- see
* https://blueprints.launchpad.net/nova/+spec/encrypt-cinder-volumes
* https://blueprints.launchpad.net/nova/+spec/encrypt-ephemeral-storage
The ConfKeyManager class reads its key from the project's
configuration file and provides this key for *all* requests. As
such, this key manager is insecure but allows the aforementioned
encryption features to be used without further integration effort.

To clarify the above statements, the configuration-based key
manager uses a single, fixed key. When used to encrypt data (e.g.,
by the Cinder volume encryption feature), the encryption provides
limited protection for the confidentiality of data. For example,
data cannot be read from a lost or stolen disk, and a volume's
contents cannot be reconstructed if an attacker intercepts the iSCSI
traffic between the compute and storage host. If the key is ever
compromised, then any data encrypted with the key can be decrypted.

This commit copies the ConfKeyManager class from Nova as well as
synchronizing changes with the key manager interface in Nova.

Implements blueprint encrypt-cinder-volumes
DocImpact
SecurityImpact

Change-Id: I5cb06386410f46cabc490fa6af23272d1d2cb979