bug: image_manifest doc needs to be updated

[okozachenko1203]

• atmosphere_images in inventory is not prior than the one in roles var file
• atmosphere_image_registry var has been removed
• after we append image digest in the atmosphere_images, crane uses those digest as the part of image tag. So pulling images using only tags from the private registry is failing because it is unknown manifest (could be the only issue with registry:2)