Kea should always return lease file location if memfile is used
This issue came up during Stork sanity checks: stork#667. The background of the problem is that Kea does not return lease file location if a default value is used. However, for an average user to determine where the lease file is located, he'd have to figure out compilation options and what prefix was configured during compilation. This is not something average user can do.
The proposal is to always return the lease file location if memfile is used, even if the location is "the default" (because effectively, the default can change from one deployment to another).