bash completion: add calls to _lttng_complete_sessions

Improve a little bit the dummy implementation of
_lttng_complete_sessions and add two calls where it can be useful.

Reported-by: Daniel U. Thibault <daniel.thibault@drdc-rddc.gc.ca>
Signed-off-by: Simon Marchi <simon.marchi@polymtl.ca>
Signed-off-by: David Goulet <dgoulet@efficios.com>