Only execute once for initial setup

### Your checklist for this issue

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- [x] Link to any upstream changes that might be required (for example Jenkins Core pull request)

### Feature Request

Please allow to execute the provided jenkins.yaml only once for the initial setup. We include this file in our Docker container but want to be able to update some configuration without having to deploy a new docker container. These manual changes are lost with each reboot of jenkins. Would be great to have a configuration option allowing to set if the CASC_JENKINS_CONFIG file should be executed on each reboot or only initially.