Use DEKStore API for Fetching and Storing passphrases.
Drop the fallback for the old KMS interface that is now provided as
DEKStore. The original implementation has been re-used for the DEKStore
interface.
This also moves GetCryptoPassphrase/StoreNewCryptoPassphrase functions
to methods of VolumeEncryption.
Signed-off-by: Niels de Vos <ndevos@redhat.com>