This adds a new `kmsConfig` interface that can be used to validate
different KMS services and setting. It makes checking for the available
support easier, and fetching the passphrase simpler.
The basicKMS mirrors the current validation of the KMS implementations
that use secrets and metadata. vaultKMS can be used to validate the
passphrase stored in a Vault service.
Signed-off-by: Niels de Vos <ndevos@redhat.com>