Retention period of task logs

[Malsourie]

Is your feature request related to a problem? Please describe.
Currently the debug logs of bosh tasks are cleaned up by ScheduledTasksCleanup module. We can configure the crontab schedule and max_tasks for the job. But the max_tasks is dependent on the platform size and hard to determine.

Describe the solution you'd like
During administration we think that it makes more sense that we have another config parameter retention_period for the job. The scheduled job will always delete the debug logs if the task is out dated.

[mvach]

Currently plan is to implement that at the end of April or beginning of May.

[Malsourie]

Feature PR: #2520
Docs PR: cloudfoundry/docs-bosh#835