fix(rolling-upgrade): Set a fixed gemini_seed for rolling-upgrades
fix(rolling-upgrade): Set a fixed gemini_seed for rolling-upgrades

The randomness of gemini cause the rolling-upgrades to be very flaky
since some workloads are heavier than others and the tests fail due
to timeouts.
This commit sets the gemini_seed to be "66" (a seed we know that
should pass according to previous runs).

scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests