DiskEncryptionKey: Encrypts the disk using a customer-supplied encryption key.
After you encrypt a disk with a customer-supplied key, you must provide the same key if you use the disk later (e.g. to create a disk snapshot or an image, or to attach the disk to a virtual machine).
Customer-supplied encryption keys do not protect access to metadata of the disk.
If you do not provide an encryption key when creating the disk, then the disk will be encrypted using an automatically generated key and you do not need to provide a key to use the disk later.