config: add allowed_values parameter to named_value constructor
config: add allowed_values parameter to named_value constructor

The _make_config_values() macro supples an optional list of allowed values
for a config item, so support that, even though no one uses it yet.

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