OpenShift: Environment variables and information about Secrets

Added necessary information about environment variable usage and about Secrets/ConfigMaps + things to make managing Secrets easier.

@ugteo link necessary parts of your example "creating an app" to/from this. Also please check if everything is understandable without years of experience with kubernetes.

Added @ntyni as a reviewer since he might have an idea if something relevant is missing as far as information goes. Not a necessicity to check now if you are busy.