improve CI / CD of the builder-clients docker image

[jstrachan]

right now we refer to different versions of the builder-clients image in various parts of the library:
https://github.com/fabric8io/fabric8-pipeline-library/search?utf8=%E2%9C%93&q=builder-clients&type=

it'd be good to have one variable for the builder clients docker image or tag so we've one place to keep the version in sync so that we can then do automated CI / CD as there are new releases of kedge / kubectl / oc / gofabric8 etc