presence: Use shared JSON config file finding & loading header

Use the shared JSON config determination header for loading its
JSON configuration file.

Tested:
    run phosphor-fan-presence-tach can find and load config.json from
    /usr/share/phosphor-fan-presence/presence and
    /etc/phosphor-fan-presence/presence/ in witherspoon qemu

Change-Id: I5c4fdf7d96de7c956ca24b4a5751911cc1bf1f03
Signed-off-by: Jolie Ku <jolie_ku@wistron.com>
4 files changed