KNOX-2989 - Multi arch support for Knox images

[moresandeep]

What changes were proposed in this pull request?

This PR:

1. Adds multi arch support for Knox images
2. Moves away from the currently used plugin that is not maintained and updates Knox docker plugin to https://github.com/fabric8io/docker-maven-plugin which is actively maintained and is feature rich.

Nothing changes as far are building process goes. What changes is now we have an ability to push multi-arch docker images to a remote repo

mvn -Ddocker.push.registry=my.docker.io/smore -U clean verify install -Prelease,package,docker  docker:push

For more details on configuration options see https://dmp.fabric8.io/#docker:push

How was this patch tested?

This patch was tested locally.