225: Add support for exclusive plans

Add new field to Plan spec, Exclusive. This field will be useful when the order of execution for a series of Jobs is important, as each exclusive Job will have to complete before the next one is scheduled.

Setting the field to true will change the default behavior of how Job Pods are scheduled: it will create a PodAntiAffinity rule which will prevent more than one exclusive labeled SUC Job Pod from being scheduled to a node at a time, and will add this label to the Job Pod.

If omitted or explicitly set to false, Job Pods will be scheduled according to current default behavior.

Implements request in rancher/system-upgrade-controller#225. @dweomer