test: Use feature_config_from_db_config to setup feature_config
test: Use feature_config_from_db_config to setup feature_config

This reduces code duplication and uses the same code path that is used
in scylla itself.

Signed-off-by: Rafael Ávila de Espíndola <1975039f9046bf513b77c0c5639a0efc25f00e7f@scylladb.com>
Message-Id: <20200403170235.113558-1-1975039f9046bf513b77c0c5639a0efc25f00e7f@scylladb.com>

scylladb/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla