Option the set MAXIMUM triggered builds for an agent

### What feature do you want to see added?

Setting maximum cap for builds triggered on one agent.when this number is reached - we remove and delete this agent .
For instance, if this number is 100, after 100 builds on the same agent,the agent will be terminated , and by design a new compute agent will show up

This behaviour could be much helpful for cases when a new image template is created ,and you don't wan't to stop the current builds in order to delete the old template agents ( think of many jobs that running periodically on the same agent and there is always running jobs on the agent which never become inactive and the retention time event will never triggered )

### Upstream changes

[#33](https://github.com/jenkinsci/google-compute-engine-plugin/pull/33)