config: adjust boost::program_options validator to work with db::string_map
config: adjust boost::program_options validator to work with db::string_map

Fixes #1320

Message-Id: <b5f0cf5db0887c55325dbf1f551658c7ce7ab5e4@scylladb.com>

duarten/scylla,scylladb/scylla,scylladb/scylla,duarten/scylla,kjniemi/scylla,scylladb/scylla,scylladb/scylla,duarten/scylla,raphaelsc/scylla,raphaelsc/scylla,avikivity/scylla,avikivity/scylla,kjniemi/scylla,kjniemi/scylla,avikivity/scylla,raphaelsc/scylla