Fix: backward relayd: path contains a leading "ust" folder

Fix: backward relayd: path contains a leading "ust" folder

Observed issue
==============

test_output_path_relayd[lttng-tools-2.13-lttng-tools-2.10-uid] from
the lttng-ivc fails on path validation for the resulting trace.

Here lttng-sessiond is 2.13 and lttng-relayd is 2.10

Traces are generated and the following hierarchy is found in the
lttng-relayd trace folder.

 lttng_home
         └── lttng-traces
*            └── ust
                 └── joraj-alpa
                     ├── auto-20210113-165054
                     │   └── ust
                     │       └── uid

Note the extra "ust" (*) in the hierarchy.

The tests expects:
 lttng_home
         └── lttng-traces
                 └── joraj-alpa
                     ├── auto-20210113-165054
                     │   └── ust
                     │       └── uid

Cause
=====

Introduced by: 5da88b0f58d7f838068037ea449ddfb25d3e85ad [1]

relayd_add_stream now suffixes the domain_name to the pathname.
This is only necessary for cases where the corresponding
lttng-relayd version is greater than 2.10. In other cases, modification
of pathname is not necessary.

Solution
========

Perform domain suffixing only for relayd > 2.10.

Known drawbacks
=========

None.

References
==========

[1] https://github.com/lttng/lttng-tools/commit/5da88b0f58d7f838068037ea449ddfb25d3e85ad

Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: If43719c0cf8285d3242d846316d7277d4f5519f1