make cleanup and pre-creation recurrence period configurable

[ZbigniewZabost]

This PR allows Jenkins operator to configure the recurrence period for cleanup and pre-creation threads.

This allows to faster cleanup and pre-creating machines, I find this setting particular useful in scenario where openstack agents are single-use (1 executor, 0 retention time) and combined with set of quick jobs (less than 1 minute or so) that are often executed. It may lead to situation when openstack plugin is not able to create new machines due to hitting limit but at the same time the already existing machines are marked for termination filling up quota. In worst case almost 10 min to get cleanup and freed up limit.

Implementation has been inspired by same issue in ec2 jenkins plugin

This PR relates to #282

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[steveames]

This would be pretty useful. Even if the jobs aren't short, when the build queue gets longer it's painful to wait 10 minutes for a VM to get removed.

[matigoal]

Hello,

Will this pull request be merged in the next releases?

Have a great day.

[simonelbaz]

Hi,

is there any reason why this PR is not merged ?
I am ready to help if necessary.