Update Jenkins E2E specs to use new Docker image and api tooling

There is a new image for Jenkins Testing with an LTS base image, which could be provided to the E2E tests with the GitLab plugin already installed.

There is also some api tooling designed for e2e arrangement. (Configuring plugins/credentials, creating and tracking jobs/builds)

Utilizing the API for the arrangement should harden the E2E specs, and provide longer term support in the case of changes in the UI.