common/config: hook in new Option config descriptions

common/config: hook in new Option config descriptions

The C++ class member fields continue to exist for
settings defined in common/legacy_config_opts.h, but
all the schema information is coming from common/options.cc
now.

The values in md_config_t::values are automatically
copied into the C++ class member fields for legacy config
options as needed.

Signed-off-by: John Spray <john.spray@redhat.com>