Update deploy-an-application.md

Review of content style and user feedback on text heaviness.

Missing:

- Mention configuring network and routes for each cluster, however another user mentioned that configuring network doesn't belong on this page? If it does, can we explain why?

- Mention image streams to be able to deploy and also why use it in Artifactory if OpenShift has a stream for that

- Remove mentions of Buildcongfig, as that will no longer be used

Author: Pilargit12