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.
diff --git a/Makefile b/Makefile
index 46adef0..9b22fe4 100644
--- a/Makefile
+++ b/Makefile
@@ -781,7 +781,7 @@
 endif
 
 ifneq ($(USE_OT),)
-include contrib/opentracing/Makefile
+include addons/ot/Makefile
 endif
 
 #### Global link options
@@ -1006,6 +1006,7 @@
 	$(Q)rm -f addons/promex/*.[oas]
 	$(Q)rm -f addons/51degrees/*.[oas] addons/51degrees/dummy/*.[oas] addons/51degrees/dummy/*/*.[oas]
 	$(Q)rm -f addons/deviceatlas/*.[oas] addons/deviceatlas/dummy/*.[oas]
+	$(Q)rm -f addons/ot/src/*.[oas]
 	$(Q)rm -f addons/wurfl/*.[oas] addons/wurfl/dummy/*.[oas]
 	$(Q)rm -f admin/iprange/iprange admin/iprange/ip6range admin/halog/halog
 	$(Q)rm -f dev/flags/flags dev/poll/poll dev/tcploop/tcploop