update root.json from the TUF repos

Steps to do it for each $repo=[tuf|tuf-unstable], remember that the keys of each repo are different:

create a virtualenv with tuf libraries:

$ virtualenv venv
$ . venv/bin/activate
$ pip install "tuf[tools]"

Get a copy of the latest repo (this is ugly, I think you should only need the old root.json file and not the whole, but I'll have to patch tuf to fix that):

$ wget -r -nH --no-parent --reject "index.html*" https://dl.bitmask.net/$repo/linux-x86_64/metadata.staged/
$ wget -r -nH --no-parent --reject "index.html*" https://dl.bitmask.net/$repo/linux-x86_64/targets/

Update the root.json, root.py is a script that will live in bitmask_bundler repo but is still pendent of review, you can fetch it from my repo:

$ wget https://github.com/meskio/bitmask_bundler/raw/feature/7303_update_root_json/tuf/root.py
$ ./root.py $repo/linux-x86_64/ root_private_key root_pub_key targets_pub_key timestamp_pub_key

Send root.json to the release manager (ivan) to regenerate the snapshot.json.

(from redmine: created on 2015-07-25, closed on 2015-08-12, relates #7303 (closed), relates #7306 (closed), relates #7313 (closed), relates #7531 (closed))