move rhel7 worker node updates entirely to openshift-ansible

It's really confusing today the role and split between openshift-ansible and the MCO (we need a FAQ for this).

Anyways here's an idea: what if we stopped the MCD running on !CoreOS systems, and instead openshift-ansible always needed to be run to apply changes from MachineConfig. It'd be openshift-ansible that'd somehow trigger drain/reboot etc.

The downside of this is e.g. if a user wants to change the kubelet config, they oc edit but then need to bounce out of Kube down to Ansible+ssh.

But the massive benefit is that it's super clear how things work - the MCO can optimize for CoreOS, openshift-ansible optimizes fully for traditional.

And with PRs like https://github.com/openshift/machine-config-operator/pull/1586/ for example - we can rely on /usr/etc for original files, and do things like make better use of OSTree.

Start by reviewing the MCD/MCO behavior and the current boundary with openshift-ansible. Read PR 1586 and issue 119 for the related assumptions. Done would require an agreed design and implementation for moving RHEL7 worker updates, including drain and reboot handling, entirely into openshift-ansible.