mender-artifact tool should be able to sign an existing artifact

mender-artifact's -k option should be able to sign an existing artifact (originally part of MEN-1018), but right now it can only sign a filesystem image. While this doesn't prevent you from signing updates, it's pretty cumbersome, since you have to extract the contents of your mender artifact first, and then repack it using all the parameters from the build.

Documentation is correct, so it's just about the complexity of calling the command.