lttng: update enable channel for UST buffer type

lttng: update enable channel for UST buffer type

- Update implementation as preparation for LTTng Tools 2.3 which
has a different default buffer type for UST.
- Add JUnit test
- Update user guide

Change-Id: I42901637579015366eb6f09a71c9e9540e1c86ef
Signed-off-by: Bernd Hufmann <Bernd.Hufmann@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/15424
Reviewed-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
IP-Clean: Matthew Khouzam <matthew.khouzam@ericsson.com>
Tested-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Tested-by: Hudson CI
Reviewed-by: Bernd Hufmann <bernd.hufmann@ericsson.com>