lttng: Add schema for LTTng MI 3.0 (LTTng 2.8)

This makes it possible to use the LTTng Control view with schema
validation enabled and LTTng 2.8. The MI service is modified to be able
to handle a different schema depending on the LTTng version.

Change-Id: I85ec15edc3f0ce942d8df8637eadba3968fef1ca
Signed-off-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/70188
Reviewed-by: Hudson CI
Reviewed-by: Bernd Hufmann <bernd.hufmann@ericsson.com>
Tested-by: Bernd Hufmann <bernd.hufmann@ericsson.com>