DEKStore is a new interface that will be used for Storing and Fetching
DEKs. The existing implementations for KMS already function as a
DEKStore, and will be updated to match the interface.
By splitting KMS and DEKStore into two components, the encryption
configuration for volumes becomes more modular. This makes it possible
to implement a DEKStore where the encrypted DEK for a volume is stored
in the metadata of the volume (RBD image).
Signed-off-by: Niels de Vos <ndevos@redhat.com>