add validation timestamp data to 'device <id> get'

[karenetheridge]

conch device <id> get lists the results of the latest validation run, but doesn't provide a timestamp, so the implication is that those results go with the latest report that was received -- which is not true if the latest report is invalid. A timestamp should be included with those validations (or maybe the validations shouldn't even be listed at all if the latest report is invalid? but the latest validation result is likely still useful/relevant).

[karenetheridge]

Note that in conch api v3 we no longer track invalid reports, but a validations-last-run-at-timestamp would still be nice to have.