lttng: clean-up printout of session output destination

lttng: clean-up printout of session output destination

This commit fixes three UX problems with lttng's list command
output.

1) The list command currently repeats the session's output location
   twice, e.g.

auto-20190326-192630 (/home/jgalar/lttng-traces/auto-20190326-192630) [inactive]
    Trace path: /home/jgalar/lttng-traces/auto-20190326-192630

2) In the case of a snapshot session, the parentheses are empty
   and the "Trace path:" line is empty, e.g.

auto-20190326-192613 () [inactive snapshot]
    Trace path:

3) The term "path" is used even though the output may be a
   network location, e.g.

auto-20190326-194856 (tcp4://127.0.0.1:5342/ [data: 5343]) [inactive]
    Trace path: tcp4://127.0.0.1:5342/ [data: 5343]

The new output omits the output location in parentheses, doesn't
print the "Trace path" line if no output is specified, and
uses the generic "output" terminology rather than "path".

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