[Code Cleanup]: Re-factor apply logic

Currently the plugin does an exec to kubectl apply for deployment. This requires that the customer has kubectl installed in their Jenkins environment. Once the k8s server-side apply function is available we should do the work to use it so that we can remove this pre-req

Server-side apply k8s issue: https://github.com/kubernetes/enhancements/issues/555

No source file or test is named. Start by locating the plugin's current kubectl apply invocation, then read the linked Kubernetes server-side apply issue. Done means applying deployments through the Kubernetes server-side apply function without requiring kubectl in the Jenkins environment.

Written by the indexing model from the issue text.