Enable Job cleanup

[atrocities]

Larger installations of Stashbot frequently have builds that take up a ton of space. While cronifying a find that looks for old builds and deletes them works, this doesn't provide for the ability to mark builds as 'keep'.

Adding a logRotator tag to the templates with some sane defaults would be nice.

<logRotator class="hudson.tasks.LogRotator">
    <daysToKeep>30</daysToKeep>
    <numToKeep>50</numToKeep>
    <artifactDaysToKeep>-1</artifactDaysToKeep>
    <artifactNumToKeep>-1</artifactNumToKeep>
</logRotator>

Ideally it would be visible on the front end as well, but this will make sense for most builds.

[terabyte]

I think that's a suitable solution for a first-pass, and then surfacing that via the UI later would be nice. Or, of course, later allowing custom templates would also surface the knob for people to tweak.