Keep track of last update data for each dependency

[renatav]

When a repository is updated, create or update a target file for each child repository, containing: branch, commit and last update date and time. These files should be updated every time TAF repo update is run, even if there are no updates. By doing this, we create some sort of a lot of updates. See cross repository timestamp specification for more details.

All of these new target files should be placed in a new subdirectory inside targets. It should be easy to create a new role only responsible for signing these targets, although defining roles is outside of the scope of this issue.

Th question here is, how to handle someone running the updater and not having th signing keys.