Enable discovery in Zigbee plugin in IoTivity's Plugin Interface module.

Enable discovery in Zigbee plugin in IoTivity's Plugin Interface module.

The PISetup() API needed to be called. It was never being called from the
sample application. Therefore no one knew how to perform the discovery.
This should unblock anyone looking to use this plugin.

Change-Id: Idcb269a48958ee3b262356a054c78ca6e0109315
Signed-off-by: Joseph Morrow <joseph.l.morrow@intel.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/15165
Tested-by: jenkins-iotivity <jenkins@iotivity.org>
Reviewed-by: Phil Coval <philippe.coval@osg.samsung.com>
Reviewed-by: Uze Choi <uzchoi@samsung.com>