fix: discover zone name from /etc/config.mesh (#36)

The zone name has traditionally been discovered by interogating
/etc/config/services. The AREDN firmware is removing this file
in favor of /etc/config.mesh/_setup.services.{dmz|nat}. This
fix updates the location where the zone name is discovered.

Signed-off-by: Gerard Hickey <[email protected]>
Co-authored-by: Tim Wilkinson <[email protected]>

Author: hickey