Check image digests

[ivan4th]

You can now specify image digests for VM images, e.g.

image: virtlet.cloud/cirros@sha256:0615af1caf345e605e27d5419f8d4c6d087b3b41ff4b979aec094ce0399f04ab

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This change is 

• [x] verify digests in PullImage
• [x] verify digests during ImageStatus
• [x] verify digests during ListImages (skip mismatching images)
• [x] e2e test