Improve scheduling around disallowed users

[shoenig]

When submitting a job, Nomad's scheduler should be able to determine whether any nodes are able to run a task, given each task's user and each client's user.denylist + user.checked_drivers config. It should also avoid scheduling onto an incompatible client. Currently the user check only happens in a task runner hook at runtime.

Add a fingerprinter && feasibility check around user compatibility.

[mikenomitch]

Just leaving an update from the team here: This seems like a good idea, but we're not sure there's a lot of demand for it. If this is an issue for you, please thumbs up/let us know, as this is helpful for us the gauge demand for this feature.

This also isn't a ton of work (though not completely trivial), so if you need this and want to open a PR, we can provide guidance.