lttng: Add clear() call before add elements to list

lttng: Add clear() call before add elements to list

In various LTTng Tracer Control core model classes, the relevant
array list is not cleared when the setter is being called. For
example, the method DomainInfo.setChannels(List chans) doesn't
clear the channel list before filling the channel list with the
new channels passed as parameter.

Change-Id: I0e5c8c4f0bd84a6de1c383131255b61c7ca94406
Signed-off-by: Bernd Hufmann <bhufmann@gmail.com>
Reviewed-on: https://git.eclipse.org/r/10731
Tested-by: Hudson CI
Reviewed-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
IP-Clean: Alexandre Montplaisir <alexmonthy@voxpopuli.im>