lttng.control: Change the short name of the LogLevelType.ALL to "All"

lttng.control: Change the short name of the LogLevelType.ALL to "All"

Change the short name of the LogLevelType.ALL to "All" instead of an
empty string. This is because when all loglevel (LogLevelType.ALL) are
enabled for a Python logger, the loglevel is set to debug but we still
want to show the user "All".

Change-Id: I18e5800bc3261c11a74e124e76517ad316e2c4a3
Signed-off-by: Bruno Roy <bruno.roy@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/79215
Reviewed-by: Hudson CI
Reviewed-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>
Tested-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>