As an authenticated user, I can create a Content unit from one or more artifacts

[nixocio]

Add test to explore creation of content units to Pulp3.

See issue 2872

In order to create a content unit, an artifact has to exist.

Then HTTP POST to /api/v3/content/file/ < content.json

content.json

  {
    "digest": "a6ad136cd5f329e3843d67dc41176cb0391e04fe23d001d00c4ec2fbc3fd9bd7",
    "path": "8eca5b27-4641-48cd-af73-bd3e6bbc4d89",
    "artifacts": {
      "8eca5b27-4641-48cd-af73-bd3e6bbc4d89": "/var/lib/pulp/artifact/a6/ad136cd5f329e3843d67dc41176cb0391e04fe23d001d00c4ec2fbc3fd9bd7"
    }
  }