Document the need to update the Jenkins master base image

[michaelsauter]

If the openshift/jenkins:2 image is not updated, rebuilding the image on top of it does not have a great effect as the Jenkins version is fixed by the base image.

FYI @gerardcl

[gerardcl]

we could create another guide type for "ODS cluster admins" (as done for ODS "admins" and "users") where we mention requirements/configs/maintenances to happen at OpenShift cluster admin level and (recommended/required) procedures.
relates to #35

[michaelsauter]

@gerardcl I would not differentiate between "admins" and "cluster admins". I would just note down when cluster permissions are required. Otherwise we have documentation spread too much.