templates/common/_base/units: disable zincati

Zincati is the update agent used on standalone Fedora CoreOS installations;
it is not used on OKD clusters.
Zincati is not shipped in RHEL CoreOS.

This adds a condition to zincati.service which it can't fullfil,
so that Fedora CoreOS nodes will not run it.

templates/common/_base/units/zincati.service.yaml

@@ -0,0 +1,6 @@
+name: zincati.service
+dropins:
+- name: mco-disabled.conf
+  contents: |
+    [Unit]
+    ConditionPathExists=/enoent