Instance pollster status

Instance pollsters are now converted to compound points. This affects the instance_[flavor, disk, ram, vcpus] metrics. Relevant tests and test files have been updated to reflect this.

fromJusts have been removed and replaced with case matches with improved logging. Code has also been git-vogued + cleaned up slightly.
 - Why does this need to be in IO?
 - Can you document the return value? I'm not sure under what circumstances this would return Nothing, [], et cetera.