The Docker Registry manifest upload should supply a Content-Type, and
guessing from the contents is the easiest we can do right now.
Also eliminate dockerutils.manifestMIMEType, it is making it too
difficult to use the returned value to be worth the extra safety.
Move the manifest computation (with v2s1 signature stripping) out of
skopeo/signature into a separate package; it is necessary in the
OpenShift client as well, unrelated to signatures.
Other Docker-specific utilities, like getting a list of layer blobsums
from a manifest, may be also moved here in the future.
