Add new build stage in CI

[darkowlzz]

This adds a new build stage in the CI where the container image is built once and pushed to docker hub. The test and deploy stages pull the same image and use it.

New docker hub repo: https://hub.docker.com/r/stosci/cluster-operator

[darkowlzz]

TravisCI env vars are not shared with pull requests from forks. The same is true for the encrypted variables https://docs.travis-ci.com/user/environment-variables/#defining-encrypted-variables-in-travisyml . We may not be able to make this work. Using a publicly open image registry is risky when the tagged release is also pushed there and pulled for final release. We can add a conditional workaround for that. I'm still looking for a way to make this work without the security risks.