aggregator needs to retain logfiles when PR fails to pass jenkins clients tests.

[hppritcha]

Per discussion on 4/1/15, it was decided at least for now that the jenkins aggregator will normally delete any logfiles associated with a PR if the jenkins clients report success. More specifically in this case, the logfiles will be deleted from the aggregators database once the PR has been closed.

In the case when one or more of the jenkins clints reported a failure, the results for the PR will be retained even after it has been closed.

If the aggregator's database grows to large using this approach, a more refined approach for retaining logfiles for failed cases will need to be considered.

[jsquyres]

Extra bonus points if, when the logfiles are deleted (e.g., for a successful build and the PR is closed), either:

1. The aggregator somehow gets Github to delete the "details" link on the PR (i.e., so there's no dangling link left), or
2. If the aggregator can't get Github to delete the "details" link, when someone clicks through to that dangling link, they get some kind of reasonable web page from the aggregator saying, "Sorry, the results for this build are no longer available, and you probably didn't care about them anyway..." (vs. a nasty/ugly 404 page).