lttng: Add utility methods to get CTF tracer versions

lttng: Add utility methods to get CTF tracer versions

It's not uncommon to check for tracer names and versions of CTF
traces. We can offer convenience methods to do so, instead of
having to parse the environment metadata every time.

Change-Id: I9c4cbc7e36d24d972889f6c4bbc16d2db852f741
Signed-off-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Reviewed-on: https://git.eclipse.org/r/51661
Reviewed-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Reviewed-by: Hudson CI