Refactor: Reduce custom logic

https://github.com/deis/jenkins-node/pull/9 updates the jenkins-node image to act more like a default Jenkins [jnlp-slave](https://github.com/jenkinsci/docker-jnlp-slave). However, due to how e2e is setup to run (see [deis/e2e-runner](https://github.com/deis/e2e-runner)), we still have some custom logic in the `Dockerfile` and `jenkins-slave` script.

Therefore, this ticket is to make updates (most likely necessary both in this and the aforementioned e2e-runner repo) to switch up the `Dockerfile` to pull from `jenkinsci/jnlp-slave` as well as removing our version of the script (delete the `rootfs` dir).

Start with the Dockerfile and the jenkins-slave script, then review how e2e-runner sets up the image. Compare the current image with jenkinsci/jnlp-slave and determine the required changes across both repositories. Done means the Dockerfile pulls from jenkinsci/jnlp-slave and the custom rootfs directory and script are removed without breaking the e2e setup.

Written by the indexing model from the issue text.