Bluetooth: Mesh: Introduce option for Node ID timeout

Some PTS test cases only work when we're advertising using Network
Identity. Using the default timeout of 60 seconds for Node Identity
will cause this test cases to fail (i.e. the PTS gives up before
Zephyr transitions to advertising from Node Identity to Network
Identity).

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