No longer compute checksum when installing images.
No longer compute checksum when installing images.

While checksums are verified when a layer is pulled from v2 registries,
there are known issues where the checksum may change when the layer diff
is computed again. To avoid these issues, the checksum should no longer
be computed and stored until after it has been extracted to the docker
storage driver. The checksums are instead computed lazily before they
are pushed to a v2 registry.

Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)

AdamOssenford/docker-pi,josemonsalve2/docker,seanmcdaniel/docker,carlye566/docker_eric,ClusterHQ/docker-plugins,ClusterHQ/docker-plugins,carlye566/docker_eric,ClusterHQ/docker-plugins,AdamOssenford/docker-pi,carlye566/docker_eric,josemonsalve2/docker,seanmcdaniel/docker,josemonsalve2/docker,AdamOssenford/docker-pi,seanmcdaniel/docker