Fix operations which interact with artifacts

Currently our ArtifactElement implementation mistakenly inherits some configuration from whatever random project might happen to be sitting in your CWD.

This branch fixes things such that:

• We store required information in the artifacts
• We use an ArtifactProject subclass when loading an ArtifactElement
  • This assures us that we do not accidentally load anything from a random project sitting in CWD

This addresses the problems raised in this comment: #845 (comment 450629592)

This branch only adds one test case which checks that environment variable dependent integration commands use the correct environment variables encoded on the artifact when running bst artifact checkout --integrate <artifact name>, as it seems we do not yet even support bst shell <artifact name> or have any other commands to properly validate this. There is much room for improvement but it is better to add features in separate follow up branches.