This makes the _instances_cores_ram_count() method only query for instances
in cells that the tenant actually has instances landed in. We do this by
getting a list of cell mappings that have instance mappings owned by the
project and limiting the scatter/gather operation to just those cells.
Change-Id: I0e2a9b2460145d3aee92f7fddc4f4da16af63ff8
Closes-Bug: #1771810