Go builder and base images override support for downstream
deployment customizations for both airshipctl and plugins.
If required, adjust the following Zuul job variables:
- docker.base_go_image
- docker.base_release_image
- docker.base_plugins_build_image
- docker.base_plugins_release_image
Closes: #515
Change-Id: Iaf4b54a353207b06c9ed6bdcae876537e73f6e44
7998615a7b