util-gem5art: Decouple artifacts from mongodb

This commit does a few things to decouple the artifacts from mongodb.
- Creates an ArtifactFileDB which stores artifacts in a json file on the
local machine
- Adds tests fro the artifact file DB
- Sets the file database to be default if pymongo isn't installed
- Extends the Artifact class to prepare for downloading artifacts from
gem5-resources

Change-Id: I1bceef94dc53c066d1c0475e79c9a1ad1f1a6202
Signed-off-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/47463
Reviewed-by: Hoa Nguyen <hoanguyen@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>
4 files changed