permit num_rows_per_pass to be set in a dedicated file

[andysouth]

Currently num_rows_per_pass that determines how many rows are generated for each table, has to be set and modified in config.yaml.

This requires searching through config.yaml for each new project and hard-coding in for each table.

If instead there acould be a single file with one column for tablename and another column for num_rows_per_pass that could be copied between projects and no edits to config.yaml would be needed.