fix(FullScanThread): Fix bad handling of 'random' option for fullscan
fix(FullScanThread): Fix bad handling of 'random' option for fullscan

The fullscan parameter has an option to get 'random' as the ks.cf for
the full scan query.
However, this one got broken as part of the previous changes.

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