fix(sct_config): fail fast when no available non-seed nodes for nemesis
fix(sct_config): fail fast when no available non-seed nodes for nemesis

When running sct with 'nemesis_filter_seeds' set to true and when 'n_db_nodes' count is equal to 'seeds_num'
nemesis cannot find node to run causing errors.
Change prevents from setting n_db_nodes equal to seeds_num when nemesis_filter_seeds is set to true

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