Currently the init_keyspace_storage() expects that the caller would
tell it where the ks directory is, but it's not nice as keyspace may
not necessarity keep its sstables in any directory.
This patch moves the directory path evaluation into storage code,
specifically to the lambda that is called for on-disk sstables. The
way directory is evaluated mirrors the one from make_keyspace_config()
that will be removed by next patch.
Signed-off-by: Pavel Emelyanov <xemul@scylladb.com>