Request for feedback: Add new fixed-lifespan cloud retention strategy.

[rhencke]

We have found a need for an alternate cloud retention strategy for our builds.

The keep-until-idle strategy is very good, but the issue we hit is that our builds are continuously running so agent never go idle. Eventually, the agents will hit some issue we must address, such as running out of disk space. While we could work to fix the agents, it's much easier to just delete them and let them be recreated.

So, we have introduced a fixed-life strategy to the vSphere plugin.
The fixed-lifespan strategy provisions an agent for a fixed amount of time. After the amount of time has expired, the agent is disconnected, any builds finished, then the agent is removed. This prevents incurring the costs of the build-once strategy, which involves bootup/provisioning/no cache costs, but ensures the agents are recycled on a regular basis which helps prevent issues.

While, currently, we do this as a separate retention strategy, I could also see this simply being an option that could be added to the existing keep-until-idle strategy.

I was hoping to get your thoughts, and if interested, submit this for inclusion.

Thank you.

[pjdarton]

This PR has been inactive for some time now... @rhencke are you still interested in completing this and getting it merged in?

I'm not keen on having PRs open for ages - I'd prefer that we either work on them until they're complete and get them merged in, or we close them if that's not going to happen.