encrypt image data for image data security

In data security consideration, the glance may need to provide the encryption feature. When the image is uploaded, the image is stored in an encrypted format. When using a image, need to get the secret key to decrypt the image . This can prevent user data from being leaked. At present, cinder has supported the encryption of the volume. The image can be used in a similar way to implement the encryption feature. The QEMU 2.6 version has supported the use of LUKS encrypted image file. When the instance using encrypted image file, nova can use QEMU to decrypt.