Support canary rollouts

[tnozicka]

Is this a bug report or feature request?

• Feature Request

What should the feature do: Allow rolling out a certain percentage of scylla nodes to a new version while keeping the rest at the old version.

What is use case behind this feature: Canary rollouts allow to test a certain percetage of nodes with a new scylla version and asses the performance, stability and other properties under load.

Additional information: Things to consider:

• racks vs. canaries - same number in every rack? rack may not have the same size
• we don't have revisions per ScyllaCluster but every StatefulSet has its own per each rack
• makes it harder to move away from StatefulSets in the future if we don't snapshot our own revisions now

cc @tzach

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