Bluetooth: tester: Fix net send command

After change 7e302979 some
MESH/NODE/NET/ tests would fail because we tried to send a message to a
non-unicast address using the device key. This patch fixes that by
defining and adding an app key for vendor model which is then used to
send network packets for these test cases.

Signed-off-by: Michał Narajowski <michal.narajowski@codecoup.pl>