common/config: move _apply_changes() calls up into ConfigProxy

common/config: move _apply_changes() calls up into ConfigProxy

see also the previous the commit, to reuse injectargs() and
set_mon_vals() in seastar's ConfigProxy, we need to move apply_changes()
call out of them, as this call is implemented differently in seastar's
ConfigProxy.

finalize_reexpand_meta() is not used by OSD at this moment, so we leave
it as it is for now.

* move _apply_changes() calls up into ConfigProxy
* move update_legacy_vals() out of _apply_changes() into the
callers of the latter. as it changes "values", in seastar's ConfigProxy,
it belongs to the first step of changing a config value

Signed-off-by: Kefu Chai <kchai@redhat.com>