Bluetooth: Mesh: Move Device UUID log to bt_mesh_prov_enable()

In some cases the application might only initialize its UUID after
calling bt_mesh_init(), e.g. in the case of deriving the UUID from the
identity address. To avoid confusing logs, only print the UUID when
actually enabling one of the provisioing bearers.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>