Exclusive group

Is your feature request related to a problem? Please describe. In !260 (merged) the possibility to have exclusive plans was introduced. This is really useful when e.g. upgrading crucial parts of the cluster but unfortunately then prevents parallelism in such upgrades, e.g. upgrading different node groups one by one. This could speed up plans in larger clusters and/or on bigger bare metal nodes with higher reboot times.

Describe the solution you'd like The exclusive flag should be extended with a string containing e.g. a exclusiveGroup. If the exclusiveGroup is unset the behavior should be the same as it is right now.

Describe alternatives you've considered Restructure the api to something like this:

spec:
  exclusive:
    enabled: true
    group: my-fancy-group