CONTRIB: move contrib/opentracing to addons/ot

This one is the last optional module to build with haproxy, so let's move
it to addons/. It was renamed to "ot" as it was the only one whose USE_*
option did not match the directory name, now this is consistent.

Few changes were required, only the Makefile, and doc were adjusted, as
the directory was already self-contained and relocatable.
78 files changed