samples: mesh_badge: fix BT mesh provisioning error

The BT Mesh Configuration Client API is used in an asynchronous manner
and the app key was not added before attempt binding it to mesh models.
This caused the binding functions to fail and resulted in a fail when
trying to to send mesh messages.

This commit registers a callback that is called when the app key is
assigned. After this callback is called, the binding functions are
called.

Signed-off-by: Jonas Remmert <j.remmert@phytec.de>