Git commits in Moodle 3.5.1 by David Balch

commit f93f81a9f6dce36a3ddc787dd49ec97516bda0e4
Author: David Balch <david.balch@conted.ox.ac.uk>
Date:   Mon, 28 May 2018 12:42:16 +0800
Tag:    v3.5.1

MDL-57742 core_lib: Make tool_consumer_instance_guid optional