As with the Cluster-level "spec.updatePolicy" field, add a similar
field at the InstanceGroup level, allowing overriding of the
cluster-level choice in each InstanceGroup.
Introduce a new value for the field ("automatic") as equivalent to the
default value applied when the field is absent. Honoring this new
value allows disabling automatic updates at the cluster level, but
then enabling them again for particular InstanceGroups. Without such a
positive affirmation, it's not possible to override a cluster-level
"external" policy at the InstanceGroup level, as there's no way to
specify positively that you want to recover the default
value. Instead, expressing the explicit "automatic" value is clear and
unambiguous.
We don't call klog.InitFlags yet, because that will cause a flag
redefinition error until we get everyone to stop using glog. That
will happen when we update to k8s 1.13.