* Move snapshot structs and functions into pkg/etcd/snapshot
* Move s3 client code and functions into pkg/etcd/s3
* Refactor pkg/etcd to track snapshot and s3 moves
* Add support for reading s3 client config from secret
* Add minio client cache, since S3 client configuration can now be
  changed at runtime by modifying the secret, and don't want to have to
  create a new minio client every time we read config.
* Add tests for pkg/etcd/s3
Signed-off-by: Brad Davidson <brad.davidson@rancher.com>