lttng.control: Fix: Allowing loglevel when enabling all loggers

lttng.control: Fix: Allowing loglevel when enabling all loggers

The user can now select a loglevel when enabling all loggers. Before
this, if the user selected "All" loggers and a loglevel an error message
would pop up.

Change-Id: I78c81202a676b05dc63fd7e95095f52180081c8c
Signed-off-by: Bruno Roy <bruno.roy@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/79436
Tested-by: Bernd Hufmann <bernd.hufmann@ericsson.com>
Reviewed-by: Hudson CI
Reviewed-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>
Tested-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>
Reviewed-by: Bernd Hufmann <bernd.hufmann@ericsson.com>