Cassandra scale down action

[kragniz]

/kind feature

Action allowing users to scale their cassandra clusters down (e.g. from 5 nodes to 3 nodes).

Plan:

1. ScaleIn action sets a field on pilots selected for removal
2. Pilots locally run nodetool drain, and report back when done
3. When all are drained, each pilot runs nodetool decommission, and report back when done
4. When all are decommissioned, run nodetool removetoken from leaderelected pilot