lttng: add-trigger: print generated trigger name

lttng: add-trigger: print generated trigger name

Print the generated trigger name when `add-trigger` succeeds. Also,
no message is emited when a trigger is successfully registered as
the command will print an error message if any error occurs.

There is also no need to parrot the trigger's name if it was specified
by the user.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I9607fbd358298b036bd533834143eb5e9d185cd0