Fixes 'wallaby inventory -c'.

Some node properties that were accessible via SAFE
evaluation in Wallaby classic are not safely available
over HTTP (specifically, last_checkin and identity_group,
both of which will fetch data from Wallaroo).  This
commit causes wallaby inventory to  eagerly fetch these
property values and freeze the node objects so that
inventory constraints written against these will not fail
on Wallaroo.