Fix Volcano Issue #920

Hi @timlinux, following last discussion, here is the new implementation of that fix. All the tests on safe and safe_qgis package are passing.

If you are OK with this method, I will visit all IFs and see if this issue replicable on other IFs. But, my concern in regard to your RFC https://github.com/AIFDR/inasafe/issues/949 is that somehow this volcano IF or other IF will be rewritten as for an instance we will have all impact layers generated from exposure layers.