Test example: improve logging, choice of ports

Test example: improve logging, choice of ports

* Turn up the logging in test-example/common.conf, to "all".

* Change the udp port numbers we use to 16900 and 16910, which
  conveniently have a "0" for "outside" and a "1" for "inside".

* Swap the order of the inside and outside sites in the sites file, so
  that the consistentliy-lower-numbered "outside" comes first.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>