Update Artifact instantiation code to use paths to files
The Artifact model can be instantiated using an open file or a path to a file on disk. The code in the ChangeSet needs to be updated to stop opening files before instantiating the Artifact. Both of the importers in the example plugin currently open a file before instantiating an Artifact. Both importers need to stop doing this and simply pass in a path to the file when instantiating an Artifact.
The ArtifactFileFiled needs to be updated to open the file in binary mode.
Specify a path instead of an open file for Artifact instantiation
closes #3113 https://pulp.plan.io/issues/3113