mon/ConfigMap: push kv parsing logic into add_option()

mon/ConfigMap: push kv parsing logic into add_option()

Push shared code into ConfigMap from mon + mgr.  Pass in a function to
fetch the Option*, since in the mon case it may come from somewhere other
than g_conf.

Signed-off-by: Sage Weil <sage@newdream.net>
(cherry picked from commit d8ccd221549cdd28702932deb148f075cb6d88b3)

Fixed conflict: `mopt.localized_name = name;` is added to add_option()
(due to 3821722e5660437298a7c0f41e1061d363090103)