Add missing instance-level variable changes to audit events
This is a follow-up for: Add project, group and ~~instance~~-level varia... (#8070 - closed)
During validation phase @ahuntsman noticed that instance level audit logging wasn't happening as expected.
Indeed, instance level variables get changed through a different service and thus aren't audited by the current approach used for group & project level variables.
Edited by Sam Figueroa