fix(manager default): scylla now uses Focal Fossa as default
fix(manager default): scylla now uses Focal Fossa as default

base image, hence need to update the default manager agent
to the correct repo.

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