Solve the dependency from the parameter concurrent_tasks

This solution is conservative, i.e. it does not change in any way the current distribution logic. Notice the addition of a test that makes the problem clear. See also the discussion in https://bugs.launchpad.net/oq-engine/+bug/1225287 and in https://docs.google.com/a/openquake.org/document/d/1iy8Cw7Zb45wXWMNJdpfxwl-5NrZ7qXVpX_rD1hYqauA/edit?usp=sharing