Support canary rollouts

[mflendrich]

Issue originally authored by tnozicka as #822

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