Comment 2 for bug 822654

I propose to change the directory structure to: upgrades/openquake/<version>/<step> and remove the schema subdirectory.

Rationale: having schema subdirectories gains us nothing if the change involves just one schema (the schema name can be in the file names) and is actively harmful if the change involves multiple schemas (the upgrade might require multiple changes to both schemas and we might need to interleave the scripts).