add checksum functions.
inject the checksum (sha256) in unit during publishing.
update importer, download file when not exist of has different checksum.
revert checksum generation in nodes publishing; revert checksum checking as part of decision to download associated file.
unused import.
unused constant.
Merge branch 'jortel-atomic-node-publish' into jortel-956839
Merge branch 'master' into jortel-956839
Merge branch 'master' into jortel-956839
Merge pull request #504 from pulp/jortel-956839

956839 - nodes only download file if does not exist on the child already.