Build multi-arch images in public travis

[edavidj]

Now that ppc64le and s390x are supported by public travis, build scripts for pushing out the multi-arch UBI images to Dockerhub should be moved into this repo to simplify the build system.

[edavidj]

Blocked by an issue with public travis s390x builds seen here. Won't be able to support zLinux until this is resolved.