BUILD: opentracing: exclude the use of haproxy variables for the OpenTracing context

BUILD: opentracing: exclude the use of haproxy variables for the OpenTracing context

Due to a recent change in the handling of haproxy variables, their use for
OpenTracing context transfer has been excluded from the compilation process.

The use of variables can be re-enabled if the newly defined variable
OT_USE_VARS is set to 1 when calling the 'make' utility.  However,
this should not be used for now as the compilation will end in error.

This change prevents the use of haproxy variables to convey the OpenTracing
context.  This means that the 'use-vars' parameter cannot be used in the
OpenTracing filter configuration for 'inject' and 'extract' operations.

An example configuration that uses this feature is in the test/ctx
directory, while the script to run that test is test/run-ctx.sh.

Then, the 'sess.ot.uuid' variable is no longer set when initializing the
OpenTracing session.  This means that this variable can still be used in
the OpenTracing configuration, but its contents will be empty.