Add OpenShift setup page for setting up the idea to production demo

Document the setup of the OpenShift cluster for our idea to production demo. Instead of documenting a production OpenShift Origin install, document using Docker to bring up a test instance of OpenShift Origin to run the demo from a local machine or DO droplet.

Provide a script for making the GitLab demo customisation for the test instance, but also document what is going on so people who have already set up a cluster can replicate the setup.

Page will be a peer of the i2p demo page in the sales handbook.

Created in response to: https://gitlab.com/gitlab-org/gitlab-ce/issues/25292