Ensure Secret Pulled Images

Give the admin the ability to ensure pods that use an image are authorized to access that image. This will culminate in changes to the IfNotPresent and Never pull policies, as the Always policy will go through an authentication check each time. When this feature is enabled, and an image in a pod request has not been successfully pulled with the given credentials (or successfully pulled in the past with no credentials), then the kubelet will consider the credentials unauthenticated. Thus even if the image is present, it may still be reauthenticated. - For IfNotPresent images, the kubelet will re-pull the image - For Never images, the image creation will fail This behavior mirrors what would happen if an image was not present on the node (as opposed to present, but yet to be authorized with the credentials present). This will be enforced for both IfNotPresent and Never policies. Kubelet configuration will give the administrator the ability to choose how images should be verified: 1. Do not allow access to any images without verification. 2. Only allow access to a specific list of pre-loaded images without verification. 3. Allow access to all pre-loaded images without verification (default behavior). 4. Allow access to all images without having to reverify credentials (previous behavior). During development, this feature will be guarded by a feature gate. * The issue and these changes improving the security posture without requiring the forcing of pull always, will be documented in the kubernetes image pull policy documentation. *