Rework the generated openshift-ansible playbook
Rework the generated openshift-ansible playbook

The `prerequisites.yml` playbook should only be explicitly run on
initial deployment to prepare the nodes. It is already included in the
scaleup playbooks for the new nodes so there is no need to include it
again. Re-running the `prerequisites.yml` playbook reconfigures the
container runtime and may cause outage, it is supposed to be run only
once.

Make update and upgrade playbooks exclusive. There is no need to run
both of them.

Add comments to clarify the intent for each playbooks.

Change-Id: I30278360fcc1ffa9bd7ce7cb77d023629fb6fa47
Closes-Bug: #1804790

dprince/tripleo-heat-templates,openstack/tripleo-heat-templates,openstack/tripleo-heat-templates,dprince/tripleo-heat-templates